Fix for https://github.com/mapcrafter/mapcrafter/issues/101

So the problem seemed to be two-fold, the "world" is different for the nether switch, but that wasn't being detected so there were problems when the max zoom levels didn't match.

The biggest problem, however, was setView() was using animation to apply its changes, so when the view's zoom level was changed or the position, the handlers would be processed too fast during the pan and zoom operation.  The 'markers.js' would cause leaflet to break if it ran during this time period.

Disabling the animations of leaflet for the view change prevented the async problem.