Better flow visualization

I've fixed some of the issues mentioned in the last pull request, esp. around displaying more complex flows. The code to display the graph is now more complex/javascript heavy, but it gets the job done.

Here is how the build mentioned in [the last pull request](https://github.com/jenkinsci/build-flow-plugin/pull/19#issuecomment-13385000) is rendered now:  

![Graph](https://f.cloud.github.com/assets/42138/188987/b03e84da-7e2c-11e2-9e83-fb16402bb494.png)

There are also more complicated cases, where a build's descendent is not in consecutive columns, for example:

![ComplicatedGraph](https://f.cloud.github.com/assets/42138/188988/bdb172e4-7e2c-11e2-8883-a6578db7785e.png) 

The lines are colored so they can be easily picked out from other lines between builds. Without the color, it makes it difficult to see where the line originates/ends.