Added unproject to CRS, along with a custom implementation for CRS.EPSG3...

There's no unproject method for CRSs.  This means, for instance with EPGS 3857, you must do some manual adjustments before being able to unproject coordinates.  For instance:

    L.Projection.SphericalMercator.unproject(new L.Point(-7963446.5, 5343756.0).divideBy(6378137))

Obviously this is only a minor annoyance, but it's a really simple change to L.CRS and L.CRS.EPSG3857.