Issue #783: python3 compatible - introduced theano.compat.

 - Included six.py by Benjamin Peterson.  (Commit 62d8be3 2013-01-13.)
   See <https://bitbucket.org/gutworth/six>.

 - Moved Python 3 compatibility code to theano.compat.
Issue #783: python3 compatible - do not instruct travis to install six.