Add text reminders

[boonrs]

• Uses twilio for text reminders
• Loops through each reminder and sends to Twilio. Twilio queues the messages and sends them at a rate of one message per phone number per second. This rate is not adjustable unless an additional phone number is added to the plan.

TODO

• [x] Get due reminders and send them
• [x] Create send sms endpoint
• [x] Refactor to reuse code for email and sms
• [ ] Clean up code

[boonrs]

@gregoryjscott This is ready for review.

[gregoryjscott]

How about a Rake task for sending the SMS reminders (similar to the one for sending emails)? Otherwise this looks ready to go.