Add support for several ways to access revisions

- Latest revision (not new):
  http://localhost:8000/enwiki/page/Animal_Farm?rev/latest/wikitext
- By MediaWiki revision ID:
  http://localhost:8000/enwiki/page/Animal_Farm?rev/453204850/wikitext
- By date:
  http://localhost:8000/enwiki/page/Animal_Farm?rev/2013-01-01T20:22/wikitext
- By uuid:
  http://localhost:8000/enwiki/page/Animal_Farm?rev/6c745300-eb62-11e0-9234-0123456789ab/wikitext

Also some clean-up.