Update utility 'audiopia-cli'.

* Add option '--buffer' to activate parallel aggregation.

* Add support for sorting.

  The 'proxy_collection' class uses RTTI to check if it's source
  aggregator is a collection (abstract_collection). If so, it uses
  the 'populate' implementation of it's source instead of the base
  implementation. The 'aggregator_factory' class forwards the aggregation-type (flag).
Update JSON unit-test.
Update 'audiopia_cli' utility.
Add Python module PyBonjour to site-packages.
Add empty Python aggregator 'itunes_xml'.