Add an option to set the working directory.

This makes it possible to simply wrap a command inside a container. This makes
it easier to use a container as an unified build environment.

Examples:

~/workspace/docker
$ docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu ls
AUTHORS		 Makefile	archive.go	   changes.go	      docker
[...]


docker  run  -v `pwd`:`pwd` -w `pwd` -i -t  ubuntu pwd
/home/marco/workspace/docker