fix: Windows Installer (MSI) to use correct program directory on x64

For an ia32 installer on a x64 host the program files should be
installed in "C:\Program Files (x86)" but for an x64 installer
on an x64 host program files should be "C:\Program Files".
Merge pull request #35 from racker/fixes/fix_msi_x64_programfiles

fix: Windows Installer (MSI) to use correct program directory on x64