agent: Add Container interface

Refactorings to support testing logging interface. @bernerdschaefer @peterbourgon 
I think this is ready for a final look-over.


On Wed, Aug 6, 2014 at 4:36 PM, Bernerd Schaefer <notifications@github.com>
wrote:

> In harpoon-agent/registry.go:
>
> >  		c.Subscribe(inc)
> >  		defer c.Unsubscribe(inc)
> >
> > +		// Then we forward the modified ContainerInstances to r.statec for reporting
> > +		// to the registry's subscribers.
> >  		for {
>
> The following does the same as the code here, but is simple enough that it
> shouldn't need any additional comments:
>
> for instance := range inc {
>   outc <- instance}
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/soundcloud/harpoon/pull/21/files#r15877452>.
>