Add simple support for L.CircleMarker

[mangelats]

Add the necessary mixins to L.CircleMarker. This allows to use L.CircleMarker as the markerClass in the options of createMarker.

Example:

this.map.editTools.startMarker(null, {markerClass: L.CircleMarker});

[mangelats]

The tests are the same as MarkerEditor.js because changing the class to L.CicleMarker should not effect any of its behavior.