Merge pull request #1 from nltk/develop

Syncing forked version of NLTK - alvations
Merge pull request #2 from nltk/develop

Syncing fork with v3.0
Merge pull request #3 from nltk/develop

Syncing fork with nltk-dev
Added grow-diag-final-and implementation to symmetrize forward and
backward word alignments.
Merge pull request #711 from alvations/develop

Added GDFA Symmetrization for nltk.align