Piper/track shift history

### What is the problem / feature ?

Wanted a nice way to track the history of a shift being claimed and/or released.

### How did it get fixed / implemented ?

Added a new model `ShiftHistory` which listens to the `pre_save` signal on shifts and tracks any changes that occur.

### How can someone test / see it ?

There is no interface for this so you'd have to look in the database.


*Here is a cute baby animal picture for your troubles...*

![polar-bears-little-cute-animal-album-701047](https://cloud.githubusercontent.com/assets/824194/2557423/a6cb8416-b701-11e3-9df7-9233fd68d91a.jpg)