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>
Nit: container indentation is one space "off".

Also, is this the *name* or *id* to filter on? Or both?

I realise this is far from final yet, but some extra info (e.g. explain that `image` also supports `image:tag`) and examples would be nice