DOT printing

Prints SymPy expression trees in [DOT format](http://en.wikipedia.org/wiki/Dot_language).

```Python
In [1]: from sympy.printing.dot import dotprint

In [2]: expr = 2*x + 3*y

In [3]: with open('tmp.dot', 'w') as f:
    f.write(dotprint(expr))
```
```
$ dot -Tpdf tmp.dot -o tmp.pdf
$ evince tmp.pdf
```
![tmp](https://f.cloud.github.com/assets/306380/164068/d9d89118-7874-11e2-8d81-27dbbf507149.png)