notification/post - Sends a notification in a channel.

Method parameters:
channel_name - The name of the channel in which the notification will be posted.
text - The text to send.

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

Expected errors:
ENTER_CHANNEL_NAME - The channel name is empty.
INVALID_CHANNEL_NAME - The channel name is invalid.
CHANNEL_NOT_FOUND - A channel with the name doesn't exist.
ENTER_TEXT - The text is empty.