Add run_tests.py to setup Django for testing.

This will set up the Django environment so the Django-specific tests can
run. If Django is not present, the setup will be skipped and the tests
will be run normally. The testsuite will skip any Django-specific tests
in this case. This simplifies running the full test suite.
Merge pull request #91 from mythmon/run-tests-helper

Add run_tests.py to setup Django for testing.