Fix bug in lemma name parsing

Lemmas containing periods were causing an exception on full name lookup: wordnet.lemma('.45_caliber.a.01..45_caliber'), wordnet.lemma('aquinas.n.01.St._Thomas'), etc. Instead of splitting on the last period, we look for two digits (the sense number) surrounded by periods.
Changed test/unit/test_wordnet.py to test/wordnet_lch.doctest
Merge pull request #2 from nltk/master

Update
Updated version number in wordnet_lch.doctest
Merge pull request #454 from nschneid/patch-4

Fix bug in lemma name parsing
Merge pull request #451 from dougalg/master

Changed test_wordnet.py to wordnet_lch.doctest