task/add - Creates a new task.

Method parameters:
text - The text of the task.
deadline_time - The Unix timestamp of the deadline of the task.
tags - A space-separated list of tags.
top_priority - Whether the task should be marked as top priority.

Returns a JSON document of the following format:
<number> - The ID of the newly created task.

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