Resource: send got_request_exception on errors

The Resource _handle_500 method will now dispatch
django.core.signals.got_request_exception during exception processing.
This makes tastypie play nicely with sentry and any other apps listening
in for exceptions.

Thanks to Andrew McCloud (@amccloud) for the patch!

NOTE: the mock library was added to the test requirements to simplify
      this and other tests.

Closes #827