Add signal to KillContainer

Comes from
Issue: https://github.com/dotcloud/docker/issues/4549
PR: https://github.com/dotcloud/docker/pull/4576
Merge branch 'add_signal_to_kill_container' of github.com:Soulou/go-dockerclient
container: add options to KillContainer
authors: add Soulou
container: remove useless fmt.Sprintf call

Really?
container: remove unnecessary tag definition for struct field