Add period item

Hey, 

I've added an event type for managing range selections. 

![Screen Shot 2013-02-23 at 18 23 24](https://f.cloud.github.com/assets/1225776/188315/1f11ff3a-7dde-11e2-81f0-516f066f0102.png)

It is quite basic and currently uses opacity for a transparent effect. I haven't tested the behavior when using it together with lots of overlapping data because it does not happen in my usecase. However, if it causes problems it might be relatively easy to fix z-indexes so that it is always behind "normal" items.

Enjoy!

Nils