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)

I originally did that, but I thought it might be nice to force the user to create a meaningful title themselves. Also, the forked board is not automatically saved (maybe it should be?) so I think this makes it more clear to the user that they still need to give it a title and save.