Simple example of using indenting / AST generation

Hi Mathias,

Thanks for this great project.

I have put together a simple hello-world example of how to use an indentation based grammar and how to generate an AST from it. 

I think it could be helpful to other people as I suspect others, like me, are currently using XML, JSON or YAML as a glorified DSL. When this gets too messy they may want to switch. But these formats are hierarchical, so they are a bit less complicated than the calculator example. So this example could help?

The patch is my original work, I license it to the parboiled project under the Apache license.

Best wishes and thanks again!

Mark