Fix tokenization of contractions

Also, improve test coverage
Use basestring instead of string_types
Fix bug with blob-string comparison
Add download_corpora_lite
* Mostly working tagger and perceptron. getting 96.8, <20 seconds.
* Fixed training method for tagger
* Clean and simplify tagger
* Added tests for perceptron tagger
* Whitespace
* Hash feature names before adding them
Add AUTHORS.rst file
* Make features strings instead of tuples
* Added comment
* Don't load model for perceptron test
* Use pickle instead of cPickle, for Python 3
* Whitespace
* Remove unnecessary unicode coercion
Add trontagger.pickle to tests/ directory

This fixes the perceptron tagger tests. It's ok to have the pickled model here since
the tests/ directory isn't included in the pypi distribution
Update .travis.yml to test all branches
Merge branch 'pr/19' into dev