Pull only the required tag
Rewrite option parsing

Also allow location of config files to be specified.
Add support for links, fix volumes, command computation

Links support not complete; think I need to restart the linking
container if the linked-to container is recreated.

Effective command of container definition needs to be computed from
image config.

Volume config was getting trashed.

Renamed lib/Docker.py to lib/DockerSyncWrapper.py to avoid
potentially-confusing naming conflict.
Docker requires all command args to be strings

Yaml thinks it's fancy, but in reality it is just dumb.
Update example config to use volume path that exists