test case for unfitted idf vector
raise ValueError for unfitted idf vector

Catches an obscure AttributeError raised by calling .transform on a TfidfTransformer (with use_idf=True) that has not learned an idf vector.
Merge pull request #1656 from rlmv/idf_diag

Fix AttributeError caused by an unlearned idf vector
Remove compute_importances parameter
What's new
FIX docstring deletions
ADD test coverage for _check_stop_list
FIX comment typo
ADD test cases to fill out VectorizerMixin coverage
ADD another VectorizerMixin test
ADD test for get_feature_names
ADD test for tfidf fit with incompatible n_features
ADD test for TfidfVectorizer attribute setters
ENH: Remove compute_importances in AdaBoost
ENH: Update feature_importances in GBRT
ENH: remove "mse" method and simplify
COSMIT
DOC: feature importances
Merge pull request #1657 from glouppe/feature-importances

[MRG] Remove compute_importances
 MV Mixin tests to CountVectorizer tests