Allow sorting of entities in get() derived stores

[tony-sull]

Right now there's no way to sort entities in the store. I skipped this on purpose for v1.0, I didn't want to paint the API into a corner with a bad sorting solution.

After playing around with it I think the best approach is to allow get to accept an optional sort function. I wasn't sure at first if sorting should be done when entities are added to the store, but that doesn't really help since one store may have two different derived stores that need to be sorted differently.