Running the tests with bytes warning enabled shows some bytes/str
mixups. Fix them all.
Some examples of mixing usage:
str(b'foo') -- calling str() on bytes
b'foo' == 'foo' -- compare str with bytes
'foo' + b'bar' -- concatenating str and bytes
* 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
