Refactor of lazy model operations

[Alex Hill]

I dealt with add_lazy_relation() a few months ago working on Mezzanine and I thought it could use some detangling.

Now that the list of pending operations is stored in the Apps class, it makes sense to put the related methods on that class as well. Running a function when a model is loaded seems an appropriate job for the app registry object.

I've introduced a more generic API, whereby a user-supplied function can be called once any number of models are ready, with those freshly-loaded models as its arguments (plus optional kwargs), a helper function for related models, and the old add_lazy_relation() reimplemented in terms of the new API with a deprecation warning.