Release 2.6.4

Final changes for release 2.6.4

* The solution has been upgraded to VS2012+ and the VS2010 solution removed.
* Various changes to allow the .NET 1.1 build to complete successfully
* Removal of some warnings

The files built for the release are available at http://nunit.org/downloads
With this line commented out, won't the RangeConstraint always throw the ArgumentException? I'm surprised the tests passed.