docs: Add :issue: support
Keep callback handles for album, artist, image, inbox, search, toplist alive

Even if the end user throws away all references to the object, the handle must
be kept alive until the complete/load callback for the object is called. If the
handle is GC-ed before the callback is called, the handle is potentially
garbage, and we may crash.

The solution chosen here is to add all handles to a package-global set,
spotify._callback_handles, and remove them again when the callback is called.

The only downside I can see with this solution is that if not all callbacks
is called we have a growing set of handle references in
spotify._callback_handles, keeping both handles and the associated objects and
callback functions alive. Hopefully this will be easy to detect/debug through
simple checks of len(spotify._callback_handles).

Fixes #121
image: Test callback when image ref is thrown away