Add logic for Dynamic Fast container slices

This pull request encodes the logic for selecting the slices based on the groups stories are assigned to in Facia tool for the Dynamic Fast container.

@stephanfowler @janua Property based testing is totally AWESOME!

@kungpochicken The logic is kinda specified in `DynamicFastTest` but if it seems a little obscure, ask me.

@sammorrisdesign Until you come up with proper names for slices I've named them after [daikaiju](http://en.wikipedia.org/wiki/Kaiju) because massive monsters are kewl.

![mothra _11_](https://cloud.githubusercontent.com/assets/1001642/4252004/d4537452-3a91-11e4-8533-46ecbb6196e1.jpg)