Print help message when no subcommand is provided

It seems there was change in behaviour of argparse in python3.3:
http://bugs.python.org/issue16308
So now we have to print the error message explicitly when no
subcommand is provided by user.
Warn when reinitializing existing .git directory

Previously only message from git: "Reinitialized existing Git repository
in ..." warned that developer initialized new package in directory
with already existing git repository. It tended to be overlooked which
resulted in wrong pushes. This change makes the warning more prominent
and sends it to stderr.