L.Proj.GeoJSON

Hi, this pull request adds `L.Proj.GeoJSON` (and `L.Proj.geoJson`).  Note that unlike the other Proj-aware layers, the CRS code is read from the JSON object directly, rather than being an argument to the constructor.  This means it is not possible to set the Proj4js definition via the CRS constructor, so it must be done manually beforehand (see example).

One nice thing about this approach is that it makes it possible to display GeoJSON stored in a different projection than the map.