Add Sync::Actions mixin to allow syncing from outside the controller.

Sync::Actions can be included into any object wishing to perform sync
publishes for a given resource. Instead of using the the controller as
context for rendering, a Sync::Renderer instance is used. Since the Renderer
is not part of the request/response/session, it has no knowledge of the
current session (ie. current_user), so syncing from outside the controller
context will require some care that the partial can be rendered within a
sessionless context. Close #23 and #26.