anchor.diff:  This function needs to send cookie headers and so must be called before any output.
   
   
  -==== validUsername(string $username) : bool
  -
  -Returns *true* if '$username' is a valid username, otherwise *false*.
  -Validity is determined by a regex loaded from the *username_regex* parameter in the *[user]* section of config files.
  -
  -
  -==== availableUsername(string $username) : bool
  -
  -Returns *false* if '$username' is a username in use already in the database, otherwise returns *true*.
  -
  -
   ==== validEmail(string $email) : bool
   
   Returns *true* if '$email' is a \'valid' email, otherwise *false*.
  
positive.code: Updates the user's *username* in the database.
  Throws a *UserException* with code *102* if '$username' is invalid, code *103* if unavailable.
  See *validUsername()* & *availableUsername()* for more details.
  
  
  ==== setPassword(string $password) : void
  
negative.code: 
  == User API
  
  === Username
  
  ==== getUsername() : string
  
  Returns the *username* of the user.
  
  
  ==== setUsername(string $username) : void
  