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
Merge branch 'master' of https://github.com/natebrennand/twilio-go into voice
moving update things to update file