Changed Index Render TwiML

I modified index.coffee to use the Twilio module to render a blank
TwiML response.
Added Subscriptions

I added a basic subscription system to index.coffee. This system is NOT
COMPLETE and is NOT READY FOR PRODUCTION! I’m adding it now as a proof
of concept, but it needs to be significantly refactored.
Added Validation for Twilio

I added validation to check that a request is legitimate before
recording a subscription.
Merge branch 'subscriptions' into development