channel/user/add - Adds a new user to a channel.

Method parameters:
id - The ID of the channel to add the user to.
username - The Zvini username of the user to add.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
CHANNEL_NOT_FOUND - A channel with the ID doesn't exist.
ENTER_USERNAME - The username is empty.
USER_NOT_FOUND - No such user with the username.
USER_IS_SELF - The user you tried to add is you.
USER_ALREADY_ADDED - The user is already added.
USER_NOT_RECEIVING - The receiver hasn't opened a connection to receive channels from you.