added get_by_natural_key() to Tenant model
added initial_multitenant.json fixture
migration required, even though model is only used for tests, becuase SOUTH_TESTS_MIGRATE = True by default (in south 0.7)
fix broken tests because user profile is automatically created via a User.post_save signal
added a middleware section to the readme
Merge pull request #1 from j0hnsmith/master

Merging master from j0hnsmith