Adds a explicit colour property to the font-awesome spinners shown
while users login/signup.
Some tidying up of the registration and login JS methods in stronger.js

* Removed some console.log() calls I must have used for debugging
* We don't need to use setTimeout now - we just re-direct the user
  once they have successfully authenticated.
* Added an error-message class to the error messages we show after a bad
  XMLHttpRequest - so the text is centered and given a white font colour