leaflet.core: Add support for overview, pan, and large zoom controls (via plugins).

Add support for overview, pan, and large zoom controls in Leaflet.core using Control.MiniMap, Control.Zoomslider, and Control.Pan Leaflet plugins.

To use these controls, one has to load the corresponding plugins first.

These plugins are not official, but are listed on http://leafletjs.com/plugins.html "Notable Leaflet Plugins" page.

this.road_tile (and this.satellite_tile) were extended a bit for usage in the MiniMap.