Make the Tropo script more driven by the API

And remove Donna's phone number
Bundle update
Add Redis to the Gemfile for persisting messages
Add sinatra-contrib for JSON support
Add REDISTOGO_URL to example env
Finish application

* Send messages via Tropo API using provided Tropo Scripting helper (tropo.rb)
* Use Redis to store recent sent messages
* Allow callback from Tropo to persist message status
* Show list of recently sent messages