Make --help work for subcommands too

E.g.:

Before:

    $ tsuru-admin user-remove --help
    Are you sure you want to remove your user from tsuru? (y/n) n
    Abort.

After:

    $ tsuru-admin user-remove --help
    tsuru-admin version 0.8.2.

    Usage: tsuru-admin user-remove

    removes your user from tsuru server.