Clarified CACHED_AUTH_PREPROCESSOR must return user

Updated README to indicate the CACHED_AUTH_PREPROCESSOR function has to return the user object.
refactored the tests into a test project, made them python3 compatible, and added classifiers to setup.py for tested versions of python
Merge pull request #5 from tclancy/master

Clarified CACHED_AUTH_PREPROCESSOR must return user
Merge pull request #6 from hwkns/master

Compatibility testing for Python 3, PyPy, and Django 1.6 & 1.7
Merge branch 'master' of https://github.com/ui/django-cached_authentication_middleware

Conflicts:
	.travis.yml