Add support for altitude

This pull requests allows set_geo and get_geo to set/get the GPSAltitude tag as well at the lat/lng. 

Unfortunately, this is not backwards compatible, because get_geo now returns a 3-tuple instead of a 2-tuple. The line `lat, lng = my_file.get_geo()` will fail with "ValueError: too many values to unpack". Instead, one should use `lat, lng, alt = my_file.get_geo()`. 

I'll leave it up to the pexif maintainer(s) whether it's worth including this in the project or not. In any case, thanks for sharing pexif; it made my life easier today.