Added first version of NSI installer script to win/openvpn.nsi

Added first version of NSI installer script to win/openvpn.nsi

This win/openvpn.nsi file is a heavily cleaned-up version of the
install-win32/openvpn.nsi file. The key differences:

- paths have been adapted to new buildsystem's requirements
- obsolete XGUI support has been removed
- unused Windows version detection has been removed
- variables specific to new build system (win/settings.in, version.m4) are
  imported
- a few new installer options have been introduced:
  - install lzo2.dll
  - install msvcr90.dll (a requirement from VS2008 builds)

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Acked-by: James Yonan <james@openvpn.net>
Signed-off-by: David Sommerseth <dazo@users.sourceforge.net>