Use server-sent events

This is the implementation of #14 for harpoon-agent. It's also the first unit test :)

For now, the previous event interfaces still exist in the agent lib, and are still implemented by the agent's internal types. @peterbourgon, should those changes take place here? Or as a part of the scheduler changes?
I think we should be grouping imports like so:

```
// ...
"testing"

"github.com/soundcloud/harpoon/harpoon-agent/lib"

"github.com/bernerdschaefer/eventsource"
"github.com/julienschmidt/httprouter"
```

That is, 1) stdlib, 2) subpackage, 3) external.