Added System.DirectoryServices because csc was failing while building with mono.
Added System.DirectoryServices because csc was failing while building with mono.

cgourlay/nantcontrib,cgourlay/nantcontrib,nant/nantcontrib,dahmage/nantcontrib,nant/nantcontrib,rmboggs/nantcontrib,nant/nantcontrib,dahmage/nantcontrib,dahmage/nantcontrib,rmboggs/nantcontrib,cgourlay/nantcontrib,rmboggs/nantcontrib