Adds TwiML App SID TODO

Hey @kwhinnery!

I added a TODO for `TwiML App SID`. I was trying to understand the error [31002](http://www.twilio.com/docs/client/errors) in the Chrome Console... then oops... it is the `TwiML App SID`.

I've also substituted erb for builder to render TwiML, I think that it is an interesting approach to render xml. I used it in [Support Roulette](https://github.com/phstc/support-roulette/blob/master/views/support_roulette_call.builder).

Cheers