Generates [Leaflet](http://leafletjs.com) layer for use with the XYZ …

[roblabs]

I found your tool for gdal2tilesp.py from this Stack Overflow post, and it immediately helped me.

I have used gdal2tiles.py in the past and found the upgrade to using XYZ and all of my cores as immensely helpful.

This PR adds in the Leaflet HTML directly from gdal2tiles.py.

[roblabs]

Speed improvements from @bolshoydi