Logging api

Refactorings to support testing logging interface. @bernerdschaefer @peterbourgon 
Done.


On Tue, Aug 5, 2014 at 2:35 PM, Peter Bourgon <notifications@github.com>
wrote:

> In harpoon-agent/registry.go:
>
> >
> > -		for subc := range r.subscribers {
> > -			subc <- state
> > -		}
> > +func (r *registry) notifySubscribers(stateChange agent.ContainerInstance) {
>
> This function only has 1 caller. Probably should just be inlined.
>
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> <https://github.com/soundcloud/harpoon/pull/21/files#r15808813>.
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