Reset settings by calling set_router or set_backends
Should make it easier to reset the router or backends in the middle of a test.
Removed redundant method
Noticed by @dpoirier - testRouterMixin._pre_rapidsms_setup() calls set_router() and invokes the superclass method, which now also calls set_router().
Merge pull request #247 from caktus/feature/reset-test-router

Reset test router or backends in one call