general code cleanup
Use the 'zoomend' event for zoom level changes
because that's the right one. zoomlevelschange only triggers at zoom
level changes due to layer changes
reenable map pan animations
use single quotes everywhere...
remember the selected base layer
always load tiles while moving
use a event to recenter the map on aircraft movements
Merge pull request #51 from denschub/leaflet-improve

Various leaflet improvements
Add Esri WorldImagery
Only load Google Maps if needed
the setting is false on default since Gmaps in Leaflet is a little bit
buggy and slow.
Merge pull request #52 from denschub/leaflet-improve

Add Esri WorldImagery tile layer, make GMaps opt-in