WIP: New experimental "graph" command

Produce a graphviz compatible graph of the parts dependencies. fdp layout option seems to produce more readable graphs in this case. Example invocation:

  buildout graph | fdp -t svg -o buildout.svg

This is not really for merge, it's a quick and dirty patch that can be interesting for others.

Also, I believe it does not handle macros correctly, some dependencies might be missing.