Merged and pushed. Thanks guys!

I made one comment on bitbucket. Feel free to address or ignore it and merge.

Thanks! Malte, do you think we can merge this?

I'm done with the review. Looks good.

I can review it, but its going to take some time. I guess I won't find time for
it this week.

Florian, can I interest you in reviewing this? It is the last missing building 
block for whole-planner experiments.

Hi Jendrik, with the volume of stuff coming in every day at the moment, I don't
want to be the bottleneck for this. If you could find another reviewer for this,
this would be great.

I have implemented this feature here:
https://bitbucket.org/jendrikseipp/downward/pull-requests/37

Can you please have a look at the diff?

If you agree with the overall design, there are at least two open questions:

- Should users be able set time and memory limits for the validator?
- How should we avoid the code duplication for finding the domain file given a 
problem file?

One idea from issue569 is to let the driver validate the found plans. The most 
important question is probably whether we want to turn on plan validation by 
default. I believe we should validate all plans by default and handle VAL just 
like the other components with its own options (i.e., add --validate and --
validate-options), but without time and memory limits.