If we can't obtain the image from the remote server, use a transparent blank instead. (This fixes low zoom levels, where tiles off to one side may not be available.) Fixes #3.
Merge pull request #5 from mikebryant/wip-3

If we can't obtain the image from the remote server, use a transparent b...
Add the ability to specify a different output format. Fixes #4.
Raise a ValidationError if we're asked for an unsupported format.
Merge pull request #6 from mikebryant/wip-4

Add the ability to specify a different output format. Fixes #4.