adding a -source flag that is passed through to "nuget push" command
as per http://docs.nuget.org/docs/reference/command-line-reference#Push_Command

This allows another way to test - the package is build and then pushed, but not pushed to nuget.org
e.g.  .\buildPackage.ps1 -push true -source "c:\temp\"
will push to local file system where you can inspect the generated package.