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/527
actionResults is no longer a nillable type:

```go

// HookContext is the implementation of jujuc.Context.
type HookContext struct {
    // ...

	// actionResults holds the values set by action-set and action-fail.
	actionResults actionResults

 // ...
}
```