schedule/send - Sends a new schedule.

Method parameters:
receiver_username - The Zvini username of the receiver.
text - The text of the schedule.
interval - The number of days in which the schedule repeats. Its value can be 2 or greater.
offset - The number of days from January 1st 1970 to the next day on which the schedule is effective.
tags - A space-separated list of tags.

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

Expected errors:
ENTER_RECEIVER_USERNAME - The receiver username is empty.
INVALID_RECEIVER_USERNAME - The receiver username is invalid.
RECEIVER_NOT_FOUND - No such receiver with the username.
RECEIVER_NOT_RECEIVING - The receiver hasn't opened a connection to receive schedules from you.
ENTER_TEXT - The text is empty.
TOO_MANY_TAGS - More than 5 tags given.