Use a signal handler instead of patching save.

References #83.

Instead of patching the save method of a tracked model class, we can use
a signal handler on post_save, which means we can still pickle our model
class.

Note we can't just listen for the signal from the class we have, but
instead listen for all post_save signals. This means we actually install
a new signal handler for each tracked model class, which fires on all
model save occurrences (and returns immediately if this handler doesn't care).

We probably could improve this to have a registry of tracked models, or
something, that allows us to just install one signal handler, and filter
according to membership.
Note FieldTracker signal rules in documentation