Refactor subcommands (ext.)

Extension of #30 

Additional changes:
- Moved commands into warp.command
- Registered functions as commands by a decorator (not usable by apps yet, because there is currently no good way to load custom code before parsing command line options)
- Fixed skeleton command not using "siteDir" option correctly ("twistd warp skeleton -d test" did not work, but "twistd warp -d test skeleton" did)