Add confirmation dialog for destructive actions

Things like deleting files or clearing your history are covered by this.
Closes #135 

@jdiez17 ready to go

![2013-11-22_18 34 29](https://f.cloud.github.com/assets/1310872/1605711/01f2bdd2-53e8-11e3-8ae4-2cc32acb2d9d.png)