output graph of images to dot (graphviz)

As a very visual person, I wanted a way to see how the image layers related to each other.  Here is my attempt at providing that.

```
$ docker graph | dot -Tpng -o docker.png
```

Here is my output, after creating a few images with the builder:

![docker](https://f.cloud.github.com/assets/8082/452443/07c3a5e8-b2e7-11e2-97b4-412371d4efa5.png)

It's pretty simple so far, with the blue boxes showing the images that have repository names.

Feedback is most certainly welcome.  I'm very new to Go (this change is about 90% of the Go that I've ever written).  Also, I plan on adding tests for this before it's done.  I just wanted to get it out there for feedback first.

Thanks.