Added support for ajax-based server side validation (useful for validation of emails, duplicate usernames, etc.)
User form more or less ready in the basic version (i.e. without permission management)
The idea is that REST handlers always fail with throwing an Error (subclass of InteroperableError). The InteroperableError contains type and data field which are JSON-serialized and sent to client. It's up to the client to interpret the error and choose an appropriate way to present it.
