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

Refactored the subcommand invocation logic to form a more hierarchical structure and a cleaner design.

Don't prefix tag names with 'v'. If you want that behaviour, include the 'v' on the command line manually.

Dynamically collect the true git exec path and only if that can not be found, use the default. Variable GIT_EXEC_PATH remains overwritable explicitly when given as a command to 'make'.

Conflicts:
	gitflow