Escape newlines in node labels for viz

I'v encountered some mangled viz output with 1.2.11:

![bad](https://f.cloud.github.com/assets/1820866/1607114/7bebb45e-546f-11e3-9ab7-50d1b0c43ee5.png) 

This trivial fix does the trick for me, I'm using clojure 1.5.1, rhizome 1.9 (same with 1.8) and graphviz 2.26.3-15 on debian.

After:
![good](https://f.cloud.github.com/assets/1820866/1607115/7bee0024-546f-11e3-99ac-ce3b30679059.png)


I'm really thrilled to have comes across this library; The API is beautifuly simple and It took me
literally minutes to learn and just a little longer to actually solve my problem, which left me several
hours to just have fun playing with my shiny new toy. You, sir, are *awesome*..