Read GPS coordinates from EXIF tags

If GPS information can be found in the EXIF tags, latitude and longitude will be extracted and added to the `media.exif.gps` variable. This should also fix #4.

Some ideas to extract the GPS coordinates from the EXIF tags where borrowed from Eran Sandler's [gist](https://gist.github.com/erans/983821).

Although I am still not sure if this can be merged from a licensing standpoint, I'd like to have this PR for further discussion.