Add rm parameter to build

Even if it's not in the documentation a rm parameter can be passed to suppress intermediate container as you can see here :

https://github.com/dotcloud/docker/blob/20f36f088c86569d59c8cfe27a99742f31f4fbd3/commands.go#L165

There is also a remote parameter which may be interesting to add in the future (but for now i don't know the purpose of this parameter)