Implement an extractor for wikimedia commons (for tropen museum)
Working proof of concept for tropenmuseum.
Try to infer image size from meta items if possible.
Finish up stuff for the Tropen museum
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
Updated Tropenmuseum docs
Merge branch 'breyten-tropenmuseum'

Conflicts:
	docs/user/datasets.rst
	ocd_backend/sources.json