Add support for needed real life features

This pull request adds two things.
1. Support for sending data without limiting to the "msg" field which is not really needed.
2. Support sending bulk messages on a queryset. Sending to an unlimited amount of objects in an efficient way.

These are needed in order to use this framework, as they are both very common real-life use cases.

Note: if you wish, this merge commit can be dropped, and you can just use the 2 commits themselves, as they are not tightly tied.
Maybe we should keep the old name `send_message`. This will help us to keep compatibility as well. Otherwise we have to add some DeprecationWarning message.