Simple GraphViz export

A very crude export mechanism to graphviz file format.

Supports taps (with explicit module name, even if first position)

Does not support other channels

Also note that some complex setups seem to break dot (but file opens up correctly in OmniGraffle)

Example rendering:
```
Stream Name  Stream Definition                 Status
  -----------  --------------------------------  --------
  bar          http --port=9898 | filter | file  deployed
  fiz          tap:bar.http > hdfs
  fiz2         tap:bar.filter > splunk
  foo          http | log                        deployed
```

![streams](https://f.cloud.github.com/assets/313494/1334156/161b6bfe-35a4-11e3-8540-ebf9db32ab95.png)