Broadcast SMS

Closes #210.

In commit 388e0c3, we added the functionality for sending mass SMS messages and verified the functionality in `spec/libs/sms_broadcaster_spec.rb`. (A little more than `assert(true)`, eh?)

In commit 631c59c, we added a view for exercising 388e0c3 through the UI, in addition to a few helper methods and HAML support (see commit messages for more detail).

![wf-bestfriends](https://f.cloud.github.com/assets/3165745/1234891/4821636c-2990-11e3-96a8-f7da821f63ba.gif)