Fix argparse error in Python 3
    
    argparse behaves differently in Python 2 and 3 when no options and
    arguments are passed to executable fedpkg. That causes no attribute
    named command is set to parsed namespace object in Python 3. Command
    line help message has to be output explicitly.
    
    Fixes #221
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>