bes: 'emit', 'log', and friends should return the sent message

For easier testing.  Running a UDP server for every test case is
silly.
Use a MagicMock connection_class for testing emit

Instead of spawning a UDP server and talking to it.  We've already
tested the UDP-serving in ConnectionTestCase.
test/test_bes.py: For Python < 3.3, fall back on an external mock

unittest.mock is new in 3.3 [1].

[1]: http://docs.python.org/3/library/unittest.mock.html
bes/django.py: Explicitly use absolute imports for Python 2
test/test_django.py: Add LogTestCase.test_log_user_anonymous_request

And the rest of the boilerplate needed to get that going.
test/test_django.py: Add LogTestCase.test_log_user_request
test/test_django.py: Add LogTestCase.test_log_user_request_override_type
test/test_django.py: Add LogTestCase.test_log_user_request_additional_data