Move educe.pdtb.corpus out from __init__
Python 3 support

Aside from a few syntactic changes, the biggest issue seems to be
that that map and filter return iterators.

Some of it is supporting 3 (latter needed for Python 3)
We need a bit of compatibility code for now because it's not yet
released, so we want to support both NLTK 2 and 3
Help pylint out a bit with multiple inheritence
Package module for educe.pdtb
Travis: reenable Python 3; skip Python 2.6