Resolved the following issues with the WikimediaCommonsExtractor:

- Removed unnecessary references to OAI
- Simplefied overall structure
- Don't request a thumbnail since we are currently only after the largest available version of the file
- Don't request pervious versions of a file, we are only interested in the most recent version
- There's a bug in the Commons API that sometimes leads to returning invalid XML when requesting the 'meta' option (we now skip 'meta')
Changes to TropenMuseumItem:

- Removed unnecessary namespaces
- Don't store date if not present
- Replaced references to Tropen Museum website with Wikipedia URL's
  because the Tropen Museum URL construction was not reliable
- Descriptions are now also extracted with the help of BeautifulSoup
- Removed unused imports