Keep the GIF properties dictionary alive until we're done

The GIF properties dictionary is, as far as we know, only retained by
the image source properties dictionary we got it from. So in order to
ensure that the GIF properties dictionary sticks around until we're done
with it, we must not release the image source properties dictionary
until we no longer need the GIF properties dictionary or any objects
obtained from it.

It seems that this is not causing any observable issues, which suggests
that there must be something else retaining the image source properties
or GIF properties dictionaries (e.g., the image source itself). But that
could well change in a future release of OS X.
Merge pull request #7 from github/ensure-gifproperties-retained

Keep the GIF properties dictionary alive until we're done