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 think that we need to leave ```index.yaml``` in .gitignore/.hgignore files. Users will receive ```index.yaml```and the necessary initial indexes when they will cloning repo, but they will not need to add ```index.yaml``` in each commit when ```index.yaml``` will be changed.