add notifcation resource

https://www.twilio.com/docs/api/rest/notification
Merge branch 'master' of github.com:natebrennand/twiliogo
add usuage stuff
Merge branch 'master' of github.com:natebrennand/twiliogo
Merge branch 'master' into usuage

Conflicts:
	NOTES.md
Merge pull request #44 from natebrennand/usuage

Usage trigger