Add the ability to duplicate (or“fork”) an artboard

I find these shortcuts really useful when I want to quick duplicate an artboard to try out a few variations.

![screen shot 2014-05-15 at 12 49 55 am](https://cloud.githubusercontent.com/assets/438160/2978974/79bff80e-dbc2-11e3-90b1-123b4cfbe7fe.png)