preparing release 3.0.0
news update
Merge branch 'release-3.0.0' into develop
Removed obsolete PunktWordTokenizer, cf https://github.com/nltk/nltk/pull/746
Removing left-over PunktWordTokenizer import
Merge pull request #748 from pquentin/develop

Removing left-over PunktWordTokenizer import
Fix PEP8 in VerbNet reader
Fix verbnet -> VerbNet
Upgrade doctest to VerbNet 3.2
Separate VerbNet into its own doctest file
Reorganize existing VerbNet tests
Raise exceptions with VerbNetError
Add a `frames` method to get all frames for a verb
Add roles to relevant frames