Completed signup procedure

How does this work?
 - User fills up form and submits
 - On click, button will fire Ajax request to server
 - Server will check if username has already been used
 - If used, next part of Javascript will get executed
 - That next part will get some json data to validate
 - If really used, send error message
 - If username is new, just redirect and start session

TODOS:
 - Write the url into main.py
 - Possible rewrite of $.getJSON because the server has already
   redirected so it cannot be executed