fixed some reference errors
more reference errors
implementation of _repr_pdf_() for Tree
hide console window
base64-encoding the pdf is required, contrary to the ipython documentation (this only works in browsers)
use png instead of pdf due to it's wider support (in the qt console and in firefox)
ideas for getting involved
documentation updates
minor doc fixes
Merge pull request #692 from jskda/pull_request.684

Pull request.684
updates for developer information
fixed tree import to avoid tree.Tree
shorten title, link to list of contributors
updated information for contributors
fixed url formatting
fix URL
new sentiwordnet corpus reader, by Christopher Potts
basic sentiwordnet functionality now working
added basic documentation for sentiwordnet
doctest fixes