Improve call graph views

I did this work a few months but totally forgot to do the pull request. This set of changes replaces the current callgraph view with one that more closely resembles the graphviz based output of xhprof-ui.

![xhgui-callgraph](https://cloud.githubusercontent.com/assets/24086/5533960/181a24f8-8a22-11e4-89f5-6127653f3583.gif)