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>
maybe some closure like
```
func isFiltered(field string, filter []string) bool {
    if len(filter) == 0 {
         return false
    }
    for _, v := range filter {
        if v == field {
            return false
       }
    }
    return true
}
```
and call like
```
if isFiltered(event.Status, eventFilters["event"]) || isFiltered(event.From, eventFilters["image"]) || isFiltered(event.ID, eventFilters["container"]) {
    return nil
}
```