Refactor SSFilterableFetchedResultsController filter updates.

[maxmeyers]

Currently, whenever there's an change in the filter in a SSFilterableFetchedResultsController, the entire table is deleted and the new results are filtered.

With this change, only objects that do not pass the new filter are deleted, objects that pass both filters are moved if necessary, and objects that only pass the new filter (but not the old one) are inserted.

Thanks for all your hard work on this project! It seriously helped my understanding and use of CoreData.

[soffes]

Thanks! Definitely a better approach.

I'll merge when I get a chance to test it. Want to make sure it's solid before releasing.

[maxmeyers]

Cool, thanks!

[maxmeyers]

I added a couple more changes.

First, in ab618ca, I added an accessor to get the unfiltered fetchedObjects. This is useful, for example, when there's a filter in place but you still want to know how many objects there are in the total list.

Secondly, in 2c5fecc, I did an overhaul of how updates to the objects in the NSFetchedResultsController are propagated to the delegate when a filter is in place. There are logical considerations for each CRUD command that depend on the filter.

[soffes]

Any chance you can write tests for this stuff? I'd love to have confidence with this junk going forward.