normalise-date clean-up

wrong path to library module (wonder how this could have worked before
…)
multiple sources now supported

now a sources.xml can contain all kinds of sources (although only RSS
is used and supported right now). also applied same invoke mechanism
where each item gets handled by its own task. should be quite scalable
now.
api methods added

- provider info (including list of news items)
- news item info
- list of news items
related to "multiple sources now supported" commit
added collection lexicon
new methods added

- some refactoring
- convert news items to requested format (json, html, xml)
- convert one news item to requested format (json, html, xml)
- convert provider info to requested format (json, xml)
- some convenience functions to create api endpoints that are used in
several places