Cleaned up ResizeableRouter + added Specs + fixed RoutedActorCellBug

* The `Resizer` implementation was already working just fine for the most part - I needed to make some changes to get it to play nice with `RouterConfig` but otherwise everything seems to be working
* Ported the `RouterSpec` from Akka - all tests currently pass
* Added a new constructor option to `RoundRobinPool` that allows you to pass in a `Resizer` without having to specify a dispatcher et al. I'm thinking we should just change the default constructor to use optional parameters instead of having a bunch of overloads.
* Fixed a bug inside `AddRoutees` inside the `RoutedActorCell` - part of the code assumed that the behavior was mutable, the rest assumed that it was immutable. Reconciled it so everything works.

There's some behavior in our Akka.Config that isn't totally consistent with TypeSafe's, and I have no idea why. I'll post a couple of line comments below.