Support TopoJson

[dmolineus]

Additional to the used geosjon data format, topo json should be supported as well.

• [x] Consider which leaflet plugin to use, probably mapbox/leaflet-omnivore
• [ ] Implement topo json encoding in PHP (or find a library for that.)

[dmolineus]

Mapbox/leaflet-omnivore support is already added to LeafletPHP and is used for ajax loading. Gonna be used.