Implements a Python 2 and 3 compatible code base.

1. The code base now runs strictly on Python 2.7, 3.3, and 3.4. Theano is not
   Python 3 compatible so this is skipped if the tests are run with Python 3.
2. The Travis CI tests have been updated to make use of conda so that Python 2
   and 3 testing is easier.
3. The core dependencies have now been bumped up to the versions available in
   Ubuntu 14.04 LTS, we will no longer guarantee support with older versions.