zpmlib: switch to YAML instead of JSON for config file

The YAML format for much cleaner and easier to use for hand-written or
hand-edited configuration files. There are two main reasons for this:

* YAML files require less quoting than a JSON file

* YAML supports comments which will allow us to guide the user much
  better than with a JSON file
doc: replace JSON with YAML
zar_tmpl: reorder keys in template

The name of a group identifies the group, so it makes sense to me to
put it as the first key.
zar_tmpl: add comments
zar.yaml: extract template from zpmlib.zpm
Merge pull request #85 from mgeisler/yaml

Switch to YAML for ZAR config file