[WIP] Add basic Twilio integration w/ 'sendtestsms' cmd.

[toolness]

This adds basic Twilio integration, with a manage.py sendtestsms command (to parallel Django's built-in sendtestemail command). Here's its example output:

To do:

• Add tests.
• Consider adding a basic abstraction layer that allows a "fake" SMS-sending service to be used for development/debugging/testing.