Add ImageOverlay that accepts bounds as projected coordinates

Adds class ```L.Proj.ImageOverlay```, that works exactly as ```L.ImageOverlay``` except that it takes the bounds in projected (map) coordinates instead of LatLngBounds. This makes it possible to add image overlays when working with coordinates systems where latitudes and longitudes do not align with the x and y axis (probably most projections). Inspired from the discussion in #67 and https://github.com/Leaflet/Leaflet/issues/2592.