Report Windows OS names, not version numbers. Fixes #2.

All current Windows client OS versions will be reported using the product
name they were sold under (e.g., "Windows Vista SP2", "Windows 7") instead
of the OS version number (e.g., 6.1.7600.0).

This code doesn't differentiate between client and server SKUs (e.g.,
Windows 8 vs Windows Server 2012) as that information is not in the
OperatingSystem class, but only available by checking whether
OSVERSIONINFOEX.wProductType == VER_NT_WORKSTATION (in GetVersionEx).
Remove "Windows" prefix from OS name.

The BugSense web UI doesn't reserve a lot of space for this value, so
making it as concise as possible improves error display. Removing
"Windows" shouldn't introduce any ambiguity for the typical WPF
application.
Merge pull request #4 from bgrainger/issue-2-windows-os-name

Report Windows OS names, not version numbers.