Amend dates and contacts

Hi,

I have added some lines to amend_package, to the effect that:

- package['extras']['date_released'] is set based on the "veroeffentlicht" entry in package['extras']['dates']

- package['extras']['date_updated'] is set based on the "aktualisiert" entry in package['extras']['dates']

- package['maintainer'] is set based on "ansprechpartner" entry in package['extras']['contacts']

I came across some weird behaviour: the value I get back from package['extras']['dates'] is sometimes a list (good), sometimes a string (bad). No idea why that is. See comment in the code.