attach support

Signed-off-by: Victor Vieux <vieux@docker.com>
fix attach

Signed-off-by: Victor Vieux <vieux@docker.com>
refactor code for exec coming later

Signed-off-by: Victor Vieux <vieux@docker.com>
Merge pull request #198 from vieux/attach_support

docker run -i -t  / attach support