Add raster utility.
Add tile coordinates to `avecado::tile` object.

This is because overzooming allows the return of a different tile
from the one which was requested and this information needs to be
passed into the renderer so that it can adjust the coordinates
relative to the requested tile (viewport).
Consolidated all instances of `box_for_tile` into one location.
Merge pull request #41 from MapQuest/render_util

Added render utility