Add coverage support

Add a .coveragerc
Run tests with 'python run_tests.py cover'
Fix bug when tagging a string with non-ascii characters
Add NLTKClassifier wrapper class

This makes it easier to add more classifiers from nltk.classify
Add DecisionTreeClassifier
Add train() and labels() methods to classifiers

Also:
- More test coverage. 95%!
- Update NLTK
Bump version 0.6.1 -> 0.6.2