Add ostree_async_progress_changed_callback()

Add the pull progress reporting function to libostree so it can be shared with rpm-ostree, as rpm-ostree's copy is a little out of date.  If you're cool with this, I have a pull request ready for rpm-ostree as well.

Still going to take a look at https://bugzilla.gnome.org/show_bug.cgi?id=740276 (if it's still relevant), but getting the easy bits done first.