Give all subcommands an optional setup() function that will be called by git-flow in order to let the subcommand initialize its environment.

Give all the branch-type subcommands a default explicit "list" action, too.

Order the functions inside each of the subcommands in a specific order, for consistency:
- usage()
- setup()
- cmd_default()
- cmd_list()
- cmd_help()
- parse_args()
- other commands