FIX regression in CountVectorizer handling of float min_df/max_df

Fixes #2595.
BF: use hasattr with providing attr name (Thanks to Timo Schulz)
BUG: fix partial_fit of MiniBatchDictLearning

The inner state of the algorithm (essentially sufficient statistics
approximating the data already seen) were not kept, and as a result
partial_fit was strongly sub-optimal
DOC: update whats_new
DOC explain ARPACK algorithm in TruncatedSVD docstring

Fixes #2572.
ENH refactor feature selection by score