Added functionality that allows pagination on the list page

Hi Leo,

This should add pagination to the list page. It fixes https://github.com/citadel-eu/agt/issues/25. There's two links (prev and next) under the <ul> and an offset is added to a few JS functions to decide which POIs are loaded. Nothing changes to the map interface.

The default number of items to show is defined in poi-lib.js, default it's 10. We might consider making this configurable in Config.php

Kind regards,
Thimo