add outer testset to docs and fix example

[ericphanson]

locally the example was failing for me. I think it could depend on the exact random stream whether or not the matrix is positive-definite, as required by quadform; here, I ensure it.

The outer testset collects handy timings for us:

Test Summary:                                                                                                          | Pass  Total     Time
Examples                                                                                                               |   32     32  1m19.5s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/DCP_analysis.jl                                              |        None     0.0s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/basic_usage.jl                                               |        None     0.1s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/chebyshev_center.jl                                          |        None     0.3s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/control.jl                                                   |        None     0.1s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/dualization.jl                                               |        None     3.7s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/huber_regression.jl                                          |        None     0.2s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/lasso_regression.jl                                          |        None     1.1s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/logistic_regression.jl                                       |        None     0.1s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/max_entropy.jl                                               |        None     0.0s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/optimal_advertising.jl                                       |        None     0.1s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/robust_approx_fitting.jl                                     |        None     0.0s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/svm.jl                                                       |        None     0.0s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/svm_l1regularization.jl                                      |        None    35.7s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/trade_off_curves.jl                                          |        None     0.8s
  /Users/eph/Convex.jl/docs/src/examples/general_examples/worst_case_analysis.jl                                       |        None     0.3s
  /Users/eph/Convex.jl/docs/src/examples/mixed_integer/binary_knapsack.jl                                              |        None     0.0s
  /Users/eph/Convex.jl/docs/src/examples/mixed_integer/n_queens.jl                                                     |   28     28     0.1s
  /Users/eph/Convex.jl/docs/src/examples/mixed_integer/section_allocation.jl                                           |        None     0.1s
  /Users/eph/Convex.jl/docs/src/examples/optimization_with_complex_variables/Fidelity in Quantum Information Theory.jl |        None     7.9s
  /Users/eph/Convex.jl/docs/src/examples/optimization_with_complex_variables/phase_recovery_using_MaxCut.jl            |        None     0.6s
  /Users/eph/Convex.jl/docs/src/examples/optimization_with_complex_variables/povm_simulation.jl                        |        None     0.2s
  /Users/eph/Convex.jl/docs/src/examples/optimization_with_complex_variables/power_flow_optimization.jl                |    4      4     0.2s
  /Users/eph/Convex.jl/docs/src/examples/portfolio_optimization/portfolio_optimization.jl                              |        None     0.0s
  /Users/eph/Convex.jl/docs/src/examples/portfolio_optimization/portfolio_optimization2.jl                             |        None     0.8s
  /Users/eph/Convex.jl/docs/src/examples/supplemental_material/Convex.jl_intro_ISMP2015.jl                             |        None     0.1s
  /Users/eph/Convex.jl/docs/src/examples/supplemental_material/paper_examples.jl                                       |        None     1.2s
  /Users/eph/Convex.jl/docs/src/examples/time_series/time_series.jl                                                    |        None     1.5s
  /Users/eph/Convex.jl/docs/src/examples/tomography/tomography.jl                                                      |        None    23.9s