Closes #308 - Display if username is taken while typing it on sign-up

Displays if the username is taken while the user is typing in the name on the sign-up form. Connects to the webmaker authentication client to determine this.
Also once you add `Sorry, that name's been snagged! Please try another.` to the template, please localize it (see http://webmaker-mobile-guide.mofodev.net/localization/README.html#how-to-localize-views-in-webmaker-mobile)