Adding twiml package

Currently only supports Say, Play, and Record verbs.
Changing Render to return a Reader instead of a string
Adding go get -t in travis install step

This way test dependencies will get installed.

Goddammit travis.
Fixing enclosing Response tag in twiml
Changing Render to RenderReader

The Render method now returns a byte slice instead of a Reader
interface.
Adding end-to-end test to twiml
Renaming Twiml struct to Response
Massive refactoring into separate files and interfaces
Adding sms verb
Moving add<verb> methods into the matching verb file
Merge pull request #6 from natebrennand/twiml

Twiml