View callgraph

Add a basic callgraph visualization. I don't think it is 100% done, but it is a start and I'd like to get this merged in before doing more work. So far the following works:

* A callgraph image is displayed.
* The balls are sized & coloured based on inclusive wall time. This makes a nice descending chain for the critical path.
* Tooltips with % wall time and function names are included.

Not done:

* Links to the symbols + run data.
* Better chart layout. I've yet to figure out how to convince d3 to do a less silly layout.

Related to #13