Improved verify

[explody]

Redoing this completely since I broke the last one.

This PR addresses a few issues in the CLI verification process, and #1079

• Stacked exceptions were yielding lots of tracebacks
  Rather than using exceptions for non-error failure states, I've updated the code to use return values (False = revoked, True = good, None = unknown). Exceptions were left in place where they are for unexpected errors (parse errors, http errors, etc.)

• Verifying a large number of certificates was causing slowdowns as the code was pulling the same CRL over and over for certs with the same CRL.
  I implemented an extremely simple cache so that an individual CRL is only downloaded once. This works fine for the CLI where the code is loaded fresh each time but it could be a problem if the verify methods are used by the running web app, as the cache has no update/expire feature. Currently, verification is only in use by the CLI code.

• Better logging
  Added debug logging output in case anyone needs to troubleshoot the process. Also, parse the cert in verify() and pass it to crl_verify() and ocsp_cerify() so they both have access to cert data for logging purposes.

Tests, flake8 and docstrings have been updated since the previous PR and master is merged in.

[explody]

Build appears to be failing in test-cli, I don't think it's related to anything in this PR. test-python locally passes fine.

[castrapel]

This appears to be an issue with lint. Try running flake8. The specific error in Travis-CI is:

--> Linting Python files
PYFLAKES_NODOCTEST=1 flake8 lemur
lemur/tests/test_verify.py:16:53: E711 comparison to None should be 'if cond is None:'
make: *** [lint-python] Error 1

[coveralls]

Coverage decreased (-0.08%) to 60.831% when pulling 79033f4 on explody:improved_verify into eb7eb6a on Netflix:master.

[castrapel]

Looks good! Will merge. Thank you