title: Cannot use MvcHtmlString/IHtmlString with RazorEngine
body: <p>I just started playing around with the RazorEngine and I'm stumbling when using a static helper method. It just produces an MvcHtmlString/IHtmlString for the template. When calling Razor.Parse(...) I get</p>
  
  <pre><code>RazorEngine.Templating.TemplateCompilationException : Unable to compile template. 
  Der Typ 'System.Web.IHtmlString' ist in einer nicht referenzierten Assembly definiert. (not referenced)
  Fügen Sie einen Verweis auf die Assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' hinzu.
  </code></pre>
  
  <p>I have added references to <code>System.Web</code>, <code>System.Web.Mvc</code>, <code>System.Web.WebPages</code>. Also tried to add them to the <em>cshtml</em> (<code>@using System.Web</code>, <code>@using System.Web.Mvc</code>). 
  But <code>@using System.Web.Mvc</code> results in <code>.Mvc</code> is not available in <code>System.Web</code>?!</p>
  
  <p>I'm trying to build the templates in <strong>NUnit</strong> Tests currently.</p>