add more NSIndexSet helpers
WIP: implement filtering
WIP: moving between filters and handling datasource changes
WIP: filtering using a mapping table and tracking deltas

- This looks like the right path, but there are a few issues:
	- Using mappings and deltas to update when the filter is changed seems logical: the source data isn't changing (no new sections)
	- Using something to handle updates when the source data is changed.  It seems like mappings and deltas would work here, but it gets very confusing very fast since sections can be inserted and things can be moved around.
- An open issue is also how to handle empty sections.  Should they be filtered out at the data level, or is this purely a presentation layer issue?  If it should be filtered out at the data level then more needs to be tracked in the filtered datasource. If it's a presentation layer issue then the table/collection view helpers can just not report titles and views.