Chisel: staircase corner and more

I propose a change in the **"Chisel Stair"** tool.
Allow removing large cubes.

![capture du 2014-11-12 03 59 04](https://cloud.githubusercontent.com/assets/401965/5004877/079fa012-6a24-11e4-8317-33a5f07ec795.png)

16 most common forms are combined in BlockStair.

![bstair](https://cloud.githubusercontent.com/assets/401965/5004880/0ed998a6-6a24-11e4-82a2-38e2663312c8.png)

We click on each cube that we want to see disappear, if the result is not in the list of 16 above, we pass on the BlockDetailled.

And we can continue to use the tool.

![stair](https://cloud.githubusercontent.com/assets/401965/5004881/11b9d5fe-6a24-11e4-95ab-d489b7f204fb.png)