Callgraph improvements

I did a few changes to the callgraph that I think make it more useful:

* Call counts are now included in popover's and in the main view.
* Nodes have a minimum size making small nodes easier to hover over.
* Popovers are better positioned, and contain more information in a saner format.
* All lines display the directional arrows now.

Before:

![screen shot 2013-12-08 at 9 54 39 am](https://f.cloud.github.com/assets/24086/1700469/410c43f2-6019-11e3-91e7-07468894fcf7.png)

After: 

![screen shot 2013-12-08 at 9 54 17 am](https://f.cloud.github.com/assets/24086/1700470/4689895c-6019-11e3-9eb8-bf4c843c52cf.png)

Refs #76