Events filtering (daemon side)

When you have tons of containers on your host, events can be hard to read.

Here is a proposal to add some filters to `docker events`
In this 1st version: `image`, `container` and `event`

In the futur, we *might* move `--since` and `--until` into the filters.

See the doc for some examples.

Signed-off-by: Victor Vieux <vieux@docker.com>
Probably clearer if we eliminate the passive voice: "If you would like to use multiple filters, pass multiple flags (e.g.,..."