Improved DOT print

Hi, 

I'd like send a pull request about enhancement of DOT print. Current implementation of DOT output is OK if the Fields length is small (only several fields are defined in the schemes). But if there are many fields being used (in real cases, the data warehouse tables always hold hundred of columns), that will make the graph very wide (but short). It's not easy to view, even with a wide screen.

Appropriately insert some "\n" to DOT file can make the graph slim and tall, much easier to view.  

Thanks,
Branky

![old](https://f.cloud.github.com/assets/614116/242697/90dfeda8-8a26-11e2-8ede-d9d1edef452e.jpg)

![new](https://f.cloud.github.com/assets/614116/242698/a8649834-8a26-11e2-971d-0665bb58380c.jpeg)