Added support for trusted users and disabling commands per-channel.

* The @switch command has been added as a built-in tool for halfops and
above to disable commands on a per-channel basis in case of spam.
* Trusted users can now login using the private-message command LOGIN
[username] [password].  From there they can also manipulate which
modules are currently loaded.
* Added a command that allows trusted users to change the command prefix
at runtime.