Assorted doc string and member name prefix changes

Still needs more work to compile under Mono.

Patch from GSerjo.
Bump ToolsVersion to 4.0
Ditto
Ditto
Ditto
Ditto
Ditto
Local.props.example should target .NET 4.0 now
System.String.IsNullOrWhiteSpace seems to have incorrect accessibility on Mono

The original code did what System.String.IsNullOrEmpty does, too, so
that's fine.
merge heads of bug26511
Upgrade NUnit to 2.6.4

Which is compatible with .NET 4.0+.
Remove old NUnit files
merge bug26511 into default