Simplify WordList string representation
Move English-specific code to its own package

Also, all base classes are now in text/base.py so that they can be shared among different
language packages
Move taggers, extractors, sentiment analyzers, parsers to text.en

However, still maintains the old API, so imports will still work.

Also, moved tree2str and filter_insignificant to utils.py
Deprecate PerceptronTagger for 0.7.0

Bump version 0.6.3 -> 0.7.0-dev
Add extension docs
Basic Wordnet integration

- Add Word::synsets() method
- Add Word::definitions() method
- Add Synset and Lemma constructors
Add Wordnet docs
Update pattern code
Add wordnet module

Removes auto-loading of wordnet corpus so importing
TextBlob remains fast
Use proper Abstract Base Classes

So that incomplete implementations fail at initialization
Fix py2/3 metaclass compatibility
Add docs for text.base
Synsets and definitions are properties, not methods
Add build_readme task
Add PositiveNaiveBayesClassifier

- Add __repr__ to NLTKClassifiers
- waffle badge
Add require_nltk_corpus decorator

Use this for methods that require nltk data.
Update NLTK
Move string behavior to StringlikeMixin
Remove unnecessary train method
Fix __str__ behavior

- Adds implements_to_string decorator that makes __str__
return unicode. This fixes print statements so the output is
as expected.