ActionComplete / ActionFailed called after Action completion

The worker now calls ActionComplete or ActionFailed on state, depending
on the success state of the Action and any errors.

The hook is set to an error state if the Action hook is missing, or if
other errors take place during hook execution.  If the Action hook sets
a fail state, the Hook will not error, rather the Action will fail.

DEPENDS: https://github.com/juju/juju/pull/651
The behavior of the uniter here should be identical to its behavior in the case of hooks.

However, since the Action is triggered via a message to the filter loop, which contains the HookInfo and Action's ID, I don't think it will come in again.  I am not entirely certain about this, I would appreciate if @fwereade could weigh in.