Add WIP gismu validator script.

PLEASE DO NOT ACCEPT MERGE  (YET)
This is a work in progress, like a code review. I'm after feedback.

What is it that we want to validate, in terms of the structure/contents of the file?
Is there a document explaining structure/contents of these .yaml files?
What would useful metrics be?

I'm guessing: no other documentation, figure it out as I go, just assume what is there should be there, and ask if I see anything weird.

- requires pyyaml: http://pyyaml.org/