Add support for setting CRS and imageOverlays

This PR adds support for: 
- passing in CRS (http://leafletjs.com/reference.html#icrs)
- imageOverlay (http://leafletjs.com/reference.html#imageoverlay)

Additionally, I added an example for using both of these.

I couldn't see a good way to test the CRS option, as there is no map.getCRS(), but if you have a suggestion let me know and I'll try to add that as well.