added stub rule.format() for API compatibility; added doctest, recommended exports from nltk.tag.brill.nltk2 and deprecated exports from nltk.tag.brill
whitespace fix: func () to func()
improved docstrings for the tagger API: rules(), train_stats(), print_template_statistics()
moved old doctest to nltk.tag.brill.nltk2; imported and reexported the most important classes for convenience
added doctests and improved documentation for public API