Develop

Add Greedy Perceptron Tagger

* Add a pure-python greedy averaged perceptron tagger
* Lightweight implementation, faster and more accurate than pattern and NLTK taggers
* Added a few unit tests, main testing via empirical evaluation
* Tests and algorithm described here: http://honnibal.wordpress.com/2013/09/11/a-good-part-of-speechpos-tagger-in-about-200-lines-of-python/
* Perceptron code can probably be useful for other things; might be worth making a first-class citizen of the package.