ENH: output processing speed in MB/s for vectorizer example

Moving common text feature extraction methods in a new base class

Hashing vectorizer work in progress
Initial work on hashing vectorizer
Add fit_transform support using the TransformerMixin + missing ABCMeta marker
Improved the clustering example with HashingVectorizer
Remove TransformerMixin from vectorizers and do a direct fit_transform alias for HashingVectorizer instead
Improve module docstring of document clustering example
COSMIT make BaseVectorizer a mixin
cosmit
Updated whats_new.rst
DOC: Started section on hashing vectorizer in narrative section
DOC: narrative doc for HashingVectorizer
DOC: typos
DOC: merged the whats new entries and add links to the narrative doc
DOC: address @mblondel's comments
ENH: measure feature extraction speed in document classification example
DOC: typos
DOC copyedit HashingVectorizer docs