Setting up test and fixtures for subgraphs.

This pull request will not make it yet.

It addresses several failures:

- sub graphs are not well supported
  - it's a one shot either _all_ belong to a subgraph or non
  - there is no option for sub graphs containing subgraphs
- the group ID is used for the subgraph label
- Group ID is numeric. Why so?
- How can we make a hierarchical structure.

The branch contains a new directory containing fixtures. This is mimicked from https://github.com/symfony/symfony/tree/master/src/Symfony/Component/Translation/Tests

By using fixtures we are able to generate the visual outputs as well (not yet implemented)

The 2 new generate files as output into fixtures/out ... I'm not sure that is the best way but it will let us generated an SVG from the 'real' output.

Please tell how we can address these several issues :)