Port from pyexiv2 to GExiv2.

pyexiv2 is unmaintained, and as long as you continue to use it, you'll never be able to port your app to Python 3. This pull request ports your app off pyexiv2 onto GExiv2, a much newer module that sports support for Python 3, making it easy to port your app to Python 3 later on. 

I apologize in advance for the sloppiness of this diff; my editor is configured to automatically strip trailing whitespace, and you had a lot of it. If you look past that and pay attention to how much simpler the gexiv2 code is compared to pyexiv2, I'm sure you'll welcome the change.

Thanks.