Squashed 'External/BuildScripts/' content from commit a0b0763

git-subtree-dir: External/BuildScripts
git-subtree-split: a0b0763d260c98919b44e621c1a4229698391c99
Merge commit '2177e77834fe52e3443bff751e2a03e8a6e4d281' as 'External/BuildScripts'
Switched Swum.NET over to the new build script.
Added a nuspec package for ABB.Swum.dll.
Added NuGet to the externals directory.
Removed old build scripts.
Updated nunit nuget packages to 2.6.3.
Added a RestorePackages target that executes prior to build & test.
Changed relative paths in the performance tests to work relative to the common build directory.