EventSpot improvements

* Add `no_events_text` parameter for shortcode (and widget setting) -
allows you to define what text is shown when there are no events.
* Added option to define `0` as the `limit` parameter to show all events
* Sanitized widget output
* Fixed: Insert events widget code would add lots of extra spaces to
the shortcode
* Added: `onlyactive` checkbox to widget (previously it was always on)