renaming LogicParser.parse to Expression.fromstring, for consistency with the rest of NLTK, cf issue https://github.com/nltk/nltk/issues/656
Modified DRT module to support fromstring-style initialisation, cf https://github.com/nltk/nltk/issues/656
added missing qualifier so that classmethod can access class variable, cf cf https://github.com/nltk/nltk/issues/656
Modified Linear Logic module to support fromstring-style initialisation, cf https://github.com/nltk/nltk/issues/656
tweak docs; fix reference to _LogicParser
Merge branch 'develop' into issue/656