Bi-Directional Infinite Scrolling Pattern Limitations

So as I work on a branch for #292 (and the feature that gave rise to it) I've run into some thought problems.

I'm working with an app using non-linear paging (keyset pagination rather than limit&offset), heavily using the new subclass of `InfinityModel` pattern, and the next feature is likely to use bidirectional loading with two InfinityLoader instances, as outlined in the readme.

- It would be good to let InfinityModel once instantiated have a link to the infinity service, so it can automatically request additional pages of results in certain scenarios.
- `afterInfinityModel` and `infinityModelLoaded` always presume that loading appending to the set. It'd be good to pass direction (and track `canLoadMore` separately, perhaps) for 'up' as well as 'down', for the bidirectlonal-loading case. Maybe there are no more results 'after' the current point, but there might be some 'before'.

Anyone have thoughts on the above? Some of this would affect interfaces somewhat...