add crs settings to wms layer

Hello, 
thanks for this awesome library. I just discovered one issue. 
In a project I am working on I had to set the crs on the wms layer, and not on the map itself. Leaflet let's you set these settings on the wms layer as well. So I put these changes into the wms- layer.
For that I had to fork g-leaflet as well (I'll send you a pull request for this library as well).

I thought maybe you're interested in my changes, so I'll send you the pull request.

br
Gerhard Schneckenleitner