Added os_entity_view_mode hook #2033

This hook lets modules control which view modes are available to different bundles.  SV list will use it when offering view modes.

Implemented the hook for events, news, and profiles to add sidebar teaser.  os_files adds some placeholders to its view modes (although I don't think we'll be keeping them - I jsut needed something to work with for now).  os also implements os_entity_view_mode_alter, which adds teaser and full to all our bundles.