Added partial support for mapping of tagsets.
Currently only works for the Universal Tagset (Petrov, Das, McDonald, 2012).
Existing support for a simplified tagset (via simplify_tags=True) is dropped.
Access new mappings via tagset='universal', optional flag for corpus reader
methods that return pos-tagged text.

https://github.com/nltk/nltk/issues/483
Merge pull request #499 from stevenbird/master

Added partial support for mapping of tagsets.