Fixed bugs with inbox_get on both backends

-inbox_get() on default backend would not return a Message, but a Inbox
object.
-the InboxSerializer would treat the RediBackend Message (a namedtuple)
like multiple models, because namedtuple is iterable.

-Added test for inbox-detail on the rest api, previous vesion of
test_retrieve() would actually test for list (it's been renamed
test_list())

-Added MessageDoesNotExist descriptions and return 404