use fmt.Fprintf instead of fmt.Fprint

fmt.Fprint does not allow format strings
remove dead code in CmdPush
kill the right containers in runtime_test
use wg.Done() isntead of wg.Add(-1)
replace unreachable returns with panics

Not only is this a more common idiom, it'll make finding bugs easier,
and it'll make porting to Go 1.1 easier.

Go 1.1 will not require the final return or panic because it has a
notion of terminating statements.
remove superfluous panic
Merge remote-tracking branch 'dominikh/minor-code-touchups'
Merge branch 'master' of ssh://github.com/dotcloud/docker