Improve Callgraph generation

I've done a few improvements to the callgraph generation. I think these changes address #82 and #83. This adds a new UI button and some controls to control both which metric is graphed and what threshold is used to exclude nodes. The new UI bit looks like:

![screen shot 2014-01-16 at 9 18 24 am](https://f.cloud.github.com/assets/24086/1931071/4f64c6b8-7eb9-11e3-80e1-6bb81fe6cab4.png)

The callgraph now uses the collapsed data which makes it more congruent with other data views like the symbol and  run view as all function calls are accounted for. This does change the shape of some graphs but since xhprof is always lossy I don't think there is much to be done there.