Avoid an IndexError when dealing with empty strings

Fixes #778
Merge pull request #779 from he7d3r/patch-1

Avoid an IndexError when dealing with empty strings
An attempt to fix #775 BLEU score returns 1 (perfect match) instead of zero
Merge pull request #782 from dimazest/bleu_no_aligment_fix

Bleu no aligment fix
stylistic changes
fix failing doctest
normalise whitespace
Merge with development
Refactoring of wsd.lesk().