Return Bounding Box of Generated Image

[gabrielmcreynolds]

The map.render function could be improved by returning the bounding box of the lat/lng coordinates for the generated image.

Specifically, for ticket #85 to be accomplished, there would have to be some way to get the bounding box of the generated image, so that the image can be cropped down to be exactly the bbox dimensions.

Another case of this being beneficial is if this image is going to be geo-referenced later, it is necessary to know the image's bounding box.