Returns a string in the format software/version (library) (platform).
Examples are:
Mozilla/4.7 [en] (WinNT; U) Mozilla/5.0 (Windows NT 6.1; rv: 12.0) Gecko/20100101 Firefox/12.0
This is then used by IIS to determine it's capabilities and can be retrieved from the: http://msdn.microsoft.com/en-US/library/system.web.httpbrowsercapabilities