rgw: Add --help support to radosgw

http://tracker.ceph.com/issues/3074 fixes #3074

The support of --help option is added through this patch.
By now, it displays the generic options usage used in radosgw.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #406 from kri5/wip-3074

rgw: Add --help support to radosgw

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>