watch string comparator, cache spec, refactor configure

closes #15 
Well, the old test case is still valid and should have passed. We haven't changed the meaning of that API. Configure with `true` should have still fired `remove` for all of those. But, it fails to do so now which means a collection view would keep gaining more items. I made some additional tweaks here: https://github.com/AmpersandJS/ampersand-subcollection/pull/28 (keeping all your commits). We can't do a full reset from within the `configure` because then we have no way to track what `add`/`remove` events will need to be fired. Take a look at what I added and see what you think.