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.

was discussed before: actually it's a good thing to have `index.yaml` under revision control. This will show you changes (whether manual or autogenerated) and allow you to hand-pick the right indexes and keeping your devs in sync...

In case several people develop features which create autogenerated indices, don't review them and move them into the manual section of the file (which is the "right way"TM) and commit those directly... yes, this might cause conflicts (even though autogenerated is sorted alphabetically and hence probably still not too bad), but the alternative is (as the userlist example shows) that it's unlikely that the deployer will go through all combinations necessary before deploying.

I think it's much better to not ignore this file and rather get a conflict than possibly ending with a defunct production due to a missing combined index.