Add board fork functionality. Fixes #65

![screen shot 2014-08-07 at 6 08 27 pm](https://cloud.githubusercontent.com/assets/1996521/3851597/928168fa-1e98-11e4-9984-4ba8c8463cd3.png)

What's the interaction like?  They click Fork, and then have to save the board and are prompted to do so with a color change on the button?  If they close after forking w/o naming the board, what happens?