adde Lesk algorithm to nltk.sem
Modified lesk.py to suit guidelines
Added lesk.doctest for nltk/sem/lesk.py
changed to 4 length tab to suit PEP8.
split variable initialization in compare_overlaps_greedy() into two
lines and modify dictionary comprehension to suit py2.6
Merge pull request #663 from alvations/develop

added Lesk algorithm to nltk.sem