Improve docstrings with :param/:type convention

I'd been looking for a way to express these!
Parser for PDTB annotations.

A pyParsing interpretation of the grammar described in Section 6
(Description of PDTB representation format) of *The Penn Discourse
Treebank 2.0 Annotation Manual*, The PDTB Research Group, 2007-12-17
Tests for the PDTB parser
Tidy up educe.pdtb.parse documentation a bit
educe.pdtb documentation tree