Support for running the test suite with pytest

I wanted to see how hard it was to convert the DRF test suite to run with py.test. What are your thoughts about switching to pytest?

This is basically what I have changed:
 * Get rid of runtests.py
 * Moved test code from rest_framework/tests and rest_framework/runtests to tests/
 * Invoke py.test from setup.py
 * Invoke py.test from Travis
 * Invoke py.test from tox
 * Changed setUpClass to be just plain setUp in test_permissions.py
 * Updated contribution guideline to show how to invoke py.test

I have not ported the runcoverage.py script - is that something that was actually used? I found some references to Hudson/XML coverage output, but I cannot find any Jenkins server that runs the tests. If that is the case and the coverage functionality is desired, let me know and I will update the PR.

(Sorry for the rather big commit, but most of the changes are the file moves and import changes)