Move contrib/docker.bash to contrib/completion/bash/docker for better organization
Add zsh completion from @felixr with permission

See https://github.com/felixr/docker-zsh-completion for the original.
See also https://github.com/dotcloud/docker/issues/2202#issuecomment-26313287

Fixes #2202
Merge pull request #2219 from tianon/completion

Reorganize contributed completion scripts to add zsh completion