Merge: nitdoc: use svg instead of png

NItdoc asks `dot` to render svg format instead of png.

Advantages:
* better performances
* less files
* better image quality
* css-able

Benchmarks against nitdoc-next (#1446):
![image](https://cloud.githubusercontent.com/assets/583144/8102371/88a6ed90-0fed-11e5-953b-cd0fa34b8f10.png)

Pull-Request: #1501
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>