implement media endpoints

blocked on my other PR for common list stuff
https://www.twilio.com/docs/api/rest/media
Merge branch 'master' into add_media

Conflicts:
	sms/constants.go
refactor into SMS
consolidate fixtures
better error if mis-diagnosed
Merge pull request #15 from natebrennand/add_media

implement media endpoints
Adding Dial verb and its nouns
Prefixing verb and noun file names for better organization.
Fixing missing Sms method on Response struct
Adding addGather method and renaming Gather field
Adding test for Dial and its nouns
Removing logging from twiml tests

Fixes #16
More sane construction of structs
Adding expected output to example