Graphviz rendering

For now, you just have to get the output into a file and run something like this:

`dot pipeline.dot -Tpng > pipeline.png`

The result would really benefit from a little formatting, but that would require recording some more information in the `RenderedNode`s (node types, etc.)

Example:
![4](https://cloud.githubusercontent.com/assets/6147371/3485750/23bf88ee-03fd-11e4-87df-d6c2c82679ac.png)