Add leaflet-rectangle dom-module

[Compufreak345]

Adds support for http://leafletjs.com/reference.html#rectangle

I decided to put northEast and southWest as attributes instead of leaflet-points or implementing LatLngBounds like they are in leaflet. Not sure if you agree with it, it fitted best for my use case.

I could not test the demo due to missing references, but it should work (please review before merging).