add ability to handle overlapping shifts

### What is the problem / feature ?

- There was a bug in our shift grouping that made overlapping shifts get organized next to each other.
- There was no support for overlapping shifts.

### How did it get fixed / implemented ?

- Added support for overlapping shifts

### How can someone test / see it ?

Create some overlapping shifts and see that the interface is ready for koda to make them look nice.

*Here is a cute baby animal picture for your troubles...*

![sxc_chicks](https://f.cloud.github.com/assets/824194/2375285/042b8594-a86a-11e3-87ee-11f15c6d0862.jpg)