resize final tile to match requested tile size in Hillup tilestache provider

Resize (if necessary) the tile returned from Hillup:Provider:renderTile to match the requested tile size.  As it was, Hillup:Provider was returning the native size of the elevation tile.  

This change, combined with https://github.com/migurski/DEM-Tools/pull/5 allows you to seed a set of 512px (retina) tiles and use them for both retina and non-retina (256px) implementations, where the 256px tiles are just downsampled from the 512px tiles.