Add anti-spam measures to SMS

[pipermerriam]

Right now someone can send SMS messages to numbers they don't own.

Need to add a note at the end of the SMS message with instructions to reply with 'STOP' if they believe they received the message in error. Probably needs a cron job to poll for these messages and add them to a blacklist.

http://twilio-python.readthedocs.org/en/latest/usage/messages.html