title: Visual Studio 2013 is not remembering references added in the solution explorer
body: <p>As mentioned in the title I am having an issue with a Visual Studio Project.</p>
  
  <p>I have decided to use Visual Studio 2013 (latest version through dream spark) to code with Unity 3D (also the latest version) as I am having issues with MonoDevelop crashing and I use Visual Studio for other stuff anyway so it is convenient.</p>
  
  <p>I have to use an external dll for my Unity game and I find every time I start a new session the dll file is gone in the references tab in the Solution Explorer (in Visual Studio) and I have to re-add it manually. Is there some sort of global references list I can add this to to prevent that issue? I've had a look and can't find anything but I also don't know what I am looking for.</p>