Tox integration added

Hey Michael,

I added the support for tox, and in doing so I did the following:

* The contents of django_silky/ and silk/tests were moved to tests/, and a symlink was created from tests/ back to ../silk so files don't need to be copied during testing
* Imports in the tests were refactored to match the new structure(including tests/__init__.py)
* Trivial modifications were made to the test settings file to accomodate different testing environments
* Updated Readme to reflect testing for django 1.7  
* .travis.yml was edited to play nicely with tox