Inventory/windows

Add the following attributes on Windows:
* BIOS vendor
* BIOS version
* CPU model
* System manufacturer

Depends on gwmi and powershell, which seems to be installed by default on Windows 2008 and 2012.

Please backport to 3.6.x for inclusion in 3.6.1.

Redmine 6378.
Should be indented two spaces less.