Library updates and some light changes...

- replace jQuery by Zepto,
- uniform key/localKey variable to uuid,
- introduce a SHA256 library to generate cleaner uuid,
- send 400 http status code instead of 418,
- do not store os/model in localStorage (get it from server if device
  already registered),
- always show os/model field but disabled them,
- remenber last user and populate the field.