[MRG] Hashing text vectorizer

New PR to wrap the `FeatureHasher` with the configurable text tokenization features to be able to extract hashed features directly from a collection of raw text files / strings.

TODO:

- <del>Update narrative documentation</del>
- <del>Update reference documentation</del>
- <del>Update classification example</del>
- <del>Update clustering example</del>
- <del>Update `doc/whatnew.rst`</del>
- <del>Add a new example for online text classification, e.g. sentiment analysis from the twitter feed using the streaming API?</del> (twitter is a bad idea because of the oauth requirement, I'll probably write another example later)