event/edit - Edits an existing event.

Method parameters:
id - The ID of the event to edit.
text - The new event text.
event_time - The new Unix timestamp of the day of the event.
start_hour - The new hour when the event starts.
start_minute - The new minute when the event starts.

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

Expected errors:
EVENT_NOT_FOUND - An event with the ID doesn't exist.
ENTER_TEXT - The new event text is empty.