Merge branch 'python-stdlib'
Improve stdlib copy/build process.

Rather than use a custom task, use the built-in task normally used to
copy content files to output. Remove the custom task.

Also add a staging target that zips up the stdlib.
Add nuspec files and nuget.exe.
Add makefiles.
Fix build issues in Visual Studio.
Update nuget.exe.
Add zip.exe.
Enable restoration of nuget packages.

NuGet is used to supply the DLR assemblies.
Remove imputs from RestoreReferences target.
Add a script to set DLR package source.

set-dlr-source uses nuget.exe to set the on-disk location of the DLR
package, for doing local development.
Better message when set-dlr-source has not been run.
Change DLR version to 1.2.0-alpha0.
Merge branch 'python-stdlib'