Look, it's 2013

Thanks, Marc Tamlyn.

Refs 493e2ea62c9ee24b8440cc029d7a9c7d94d6e9f9.
Merge remote-tracking branch 'official/master'

* official/master:
  FeinCMS v1.6.6
ct_tracker: Do not call invalidate_cache if the base model does not have it
fix medialibrary admin to work with new extension mechanism
add documentation for extension mechanism and mediafile admin widgets
Merge remote-tracking branch 'upstream/master'