basic version of the /events endpoint

Fixes #1167 

Initial version of the /events endpoint
It uses JSON streaming, like `pull` or `push`

use `docker events` or call `GET /events HTTP/1.1` to try

List of supported events:

* kill : container kill
* export : container export
* create : container create
* restart : container restart
* destroy : container destroy
* delete : image delete
* untag : image untag
* start : container start
* stop container stop

It supports polling, you can use `since=<timestamp>` to get previous events. For now only the last 64 last events are stored.