schedule/add - Creates a new schedule.

Method parameters:
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:
<number> - The ID of the newly created schedule.

Expected errors:
ENTER_TEXT - The text is empty.
TOO_MANY_TAGS - More than 5 tags given.