setup.py: Update url and download_url to point to the thumbor fork

The fork may go away once the required changes are released upstream
[1,2], but while this is on PyPI it makes sense to point folks at the
source behind the forked package.

[1]: https://github.com/bennoleslie/pexif/issues/4
[2]: https://github.com/thumbor/thumbor/issues/294
Merge pull request #1 from wking/fork-metadata

setup.py: Update url and download_url to point to the thumbor fork