title: Add Files programmatically to VS Project for UnitTests
body: <p>I am creating Class Files programmatically for UnitTests with NUnit. 
  I could already add them to TFS Source Control.
  What I now need, is that I want to add them programmatically to the Project.</p>
  
  <p>I have found several approaches, but I got none working... I don't want to edit the XMl-File manually.</p>
  
  <p>Do you have some code snippets?</p>