client/model: Allow for variable arguments to be passed to callbacks

This patch allows a user of ari-py to pass arguments (*args, **kwargs) to
callback functions registered for events. This is particularly useful when
maintaining state; the current state can be passed around the callbacks as
opposed to being maintained globally.

Note that passing said arguments is completely optional.
Merge pull request #7 from samuelg/arbitraryparams

Allow for variable arguments to be passed to callbacks