Mark Cython outputs as binary so their changes don't clutter diffs.

.gitattributes

@@ -0,0 +1,20 @@
+/sklearn/__check_build/_check_build.c -diff
+/sklearn/_hmmc.c -diff
+/sklearn/cluster/_hierarchical.c -diff
+/sklearn/cluster/_k_means.c -diff
+/sklearn/datasets/_svmlight_format.c -diff
+/sklearn/ensemble/_gradient_boosting.c -diff
+/sklearn/linear_model/cd_fast.c -diff
+/sklearn/linear_model/sgd_fast.c -diff
+/sklearn/neighbors/ball_tree.c -diff
+/sklearn/svm/liblinear.c -diff
+/sklearn/svm/libsvm.c -diff
+/sklearn/svm/libsvm_sparse.c -diff
+/sklearn/tree/_tree.c -diff
+/sklearn/utils/arraybuilder.c -diff
+/sklearn/utils/arrayfuncs.c -diff
+/sklearn/utils/graph_shortest_path.c -diff
+/sklearn/utils/murmurhash.c -diff
+/sklearn/utils/seq_dataset.c -diff
+/sklearn/utils/sparsefuncs.c -diff
+/sklearn/utils/weight_vector.c -diff