Try to detect 64-bit version of Windows.

If 64-bit version is not detected, display a warning at the top of the download page.

Detection should work with all modern browsers: Internet Explorer (7 through 10), Chrome and Firefox.

One note about Chrome, the [check may fail on very old 32-bit versions](https://code.google.com/p/chromium/issues/detail?id=56515) (older than 2011) running on 64-bit operating systems.