Change CopyToOutputDirectory to PreserveNewest

Change csproj all CopyToOutputDirectory from Always to PreserveNewest.
PreBuildTask catch nodejs stderr

Fix in PreBuildTask.ExecWithOutputAsync(). Originally it declare var
output but not using it. I found running node npm will output to sterr,
so change the output into error, and return the right error message from
it.
Merge pull request #745 from ChrisTorng/master

Refine build related issues