EXIF.py crashes on Python3

EXIF.py tries to sort the output of {}.keys()
On Python2, {}.keys() returns a list
On Python3, {}.keys() returns a dict_keys, without a .sort() method

Wrapping the {}.keys() in list() seems to solve the problem. No negative side-effects noticed or expected.
Merge branch 'python3-quick-fix' of git://github.com/leprechaun/exif-py into leprechaun-python3-quick-fix