Stemming

[tyarkoni]

Adds a new WordStemmingFilter (and our first Filter, period!) that wraps the nltk.stem module. First argument is the name of an nltk stemmer (defaults to 'porter', for PorterStemmer); subsequent arguments are passed on to the selected stemmer's init.

[coveralls]

Coverage increased (+1.0%) to 80.405% when pulling cfa67fe on stemming into 8c2059a on master.

[tyarkoni]

Tests are failing on travis-ci on Python2, but it's a timeout in the TensorFlow inception model download, and everything's passing locally, so merging anyway.

[coveralls]

Coverage increased (+1.0%) to 80.405% when pulling 7b6e66b on stemming into 8c2059a on master.