Merge pull request #51 from denschub/leaflet-improve

Various leaflet improvements
Merge pull request #52 from denschub/leaflet-improve

Add Esri WorldImagery tile layer, make GMaps opt-in
add and remove google layers without reloading the page
fix active base layer after page load
fix tab ;)
Merge pull request #53 from SuperTux88/leaflet-improve

Google-Maps switch without page reload
A little bit of code beautification