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)

It actually breaks without copying the cache. Specifically because `panel.cache.visualization.meta.readableTitle` is missing. Maybe that itself is just bad design. I was originally thinking "yeah! just make a carbon copy of the whole session" but I suppose there may be ways for that to go wrong as well.