- In both ManyRelatedField, provide an empty return when trying to
access a relation field if the instance in question has no PK (so
likely hasn't been inserted yet)
- Add relevant tests
- Without these changes, exceptions would be raised when trying to
serialize the uncreated models as it is impossible to query
relations without a PK
- Add test to ensure RelatedField does not regress as currently
supports being serialized with and unsaved model
* 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
