option to buffer Overlay image

[sheppard]

In L.WMS.Overlay, it would be useful to be able to specify a geographic (or pixel) buffer around the WMS request to avoid any edge artifacts and provide a nicer panning experience. The image requested would thus be larger than the viewport size - potentially much larger if a geographic buffer is used. This is roughly analogous to (but not quite the same as) the keepBuffer option in L.GridLayer and L.TileLayer.WMS.

This could be implemented in conjunction with #11.

[mdartic]

An idea would be to add another option in L.WMS.Overlay, and we could use it when getting the bounds of the map.

http://stackoverflow.com/questions/34137648/leaflet-bounds-with-padding explain it.

The method pad is in my opinion the solution ?