Add support for notifications via SMS message with Twilio API Edit

Hi all,

I thought it would be nice to have support for sending snatch/download notifications via SMS messages, so I coded it up using Twilio's API.  The implementation is basically a copy of the Boxcar notification provider, which seemed the closest in functionality, except with a few extra fields.

![Sick Beard - alpha master - Config - Notifications - Google Chrome_002](https://f.cloud.github.com/assets/91093/165887/5140b326-797d-11e2-957d-ee9604f7eb9c.png)

(This is an updated pull request based upon the development branch, and with a single squashed commit, as requested from the original pull request #592)