Added command-line option parser and an unsigned build option to build_all.py

Added command-line option parser and an unsigned build option to build_all.py

Modified win/build_all.py so that it parses command-line options using getopt.
Added option "-u / --unsigned" which allows forcing unsigned builds and a "-h /
--help" option. By default a signed build is generated, provided that the Python
SignTool module is installed. If not, the build is interrupted.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Acked-by: Peter Stuge <peter@stuge.se>
Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>