Assume group numbers have the same country code as us, if none is specified

I moved number verification to helper.js, so that group messages and one-on-one messages use the same logic. 

Are group messages working right now? Whenever I try and send one, the recipient gets an error on [this line of background.js](https://github.com/WhisperSystems/TextSecure-Browser/blob/master/js/background.js#L168). `conversation.save()` returns false, and any success and error callbacks that I add are never called. I've reinstalled the app, gone back a week in commits, and nothing seems to help.