pre-populate index.yaml

Minimal index definitions for vanilla gae-init are used so the zigzag
algorithm can do the rest. Tested with the changes introduced by #201 and #215.

implements #208, closes #210 and closes lipis/gae-init#6.

I would really liked what @gmist said, but my tests showed me that if the `index.yaml` is in the repo even if you add it to the ignore list there are unstaged files after changing them.. :/ is it possible to achieve it?