Force graph

Update the callgraph visualization to use a force directed graph. This allows us to show cyclic call stacks and better visualize things the current solution cannot. In addition the this new graph is far more compact than before.

The new graph allows you to drag nodes around to help expose the critical path and explore the graph more easily. I screwed around with the various force graph options to try and get an 'always nice' graph but was largely unsuccessful. I think the results are an improvement over what currently exists and should solve #33 

Screenshot

![new callgraph](http://cl.ly/image/1h3C183E0J2r/Screen%20Shot%202013-04-30%20at%2011.10.47%20PM.png)