Python 3 support

Support for python 3. 
Tested with python 2.6, 2.7, 3.1, 3.2, 3.3
Dropped support of python 2.5 (it's ancient).
Minor cleanups in favor of pep8.

Added tox [travis tests](https://travis-ci.org/coagulant/embedly-python/builds/3779815)