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.
It's better to take a declarative approach here; instead of editing the innerHTML of the of the errBox, add your message to the element and show/hide it based on a value on the model (`this.$data`) with [v-if](http://vuejs.org/api/directives.html#v-if).

What you might do here is create an `errors` object on the model and set `errors.usernameTaken` to `true`/`false` as needed. Make sense?