Add translation API and other 0.5.0 improvements

- Moar docs
- sentiment now returns a float not a tuple. Also add subjectivity property
On second thought, keep sentiment API the same, but add new properties
Add sentiments module.

Includes a new NaiveBayesAnalyzer class.
Also remove some redundancy in np_extractors.
Raise MissingCorpusException if movie_reviews isn't found
Linting and such.

Keepin the codes smellin so fresh and so clean clean
Update nltk

Along with minor docs changes
Bumpversion 0.4.0 -> 0.5.0