add *-count of *-list elements

One solution for #101 without backwards incompatible changes and without complex/intransparent data structures.

This adds *-count for every *-list element
on the same level if available as attribute.

Example:
```
<release-list count="42">
  <release>...</release>
  ...
</release-list>

->

{"release-count": "42",
 "release-list": [...]}
```