Validate input username

Greetings,

This is commit addresses #35.  I added a method to validate the username coming in to make sure the google username is used instead.

If the user has a google apps domain (something like ```jstudent@my.wheaton.edu```), the full username should be used.  Basically, just checks the string for ```@gmail.com```.  Same rules as logging gmail.

Let me know if I'm missing something or not following your standards.  This was just an impulsive commit to a project I *just* found.  I would love to work on this and help out.  I love hangouts, but the Google client looks like crap in tiling WMs.