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
allow other extension after datepublisher to use add_extension_options()
Merge branch 'master' into template_upgrades