Refine announcement behavior and interface

Pass a lambda, not a hash, to default_scope
  Resolves a deprecation warning

Render announcement list into correct partial

Refining announcement listing structure

Announcement styling

Remove announcement with AJAX callback

Applying bootstrap styles to announcement management

Fixing announcement specs

Redirect back to the announcement list after create/update
Merge pull request #67 from ndlib/refining-announcements-system

Refine announcement behavior and interface