title: npoi 'Sheet' type or namespace could not be found
body: <p>This seems like a super simple issue to resolve.  The type or namespace 'Sheet' is simply not there, so I get the red underline with the above message.</p>
  
  <p>I have included:</p>
  
  <pre><code>using NPOI.HSSF.UserModel;
  using NPOI.HPSF;
  using NPOI.POIFS.FileSystem;
  using NPOI.SS.UserModel;//(i think it should be in here)
  </code></pre>
  
  <p>Code that has error:</p>
  
  <pre><code>HSSFWorkbook hssfworkbook;
  hssfworkbook = new HSSFWorkbook();
  
  Sheet sheet1 = hssfworkbook.CreateSheet("Sheet1");  //this causes the error
  </code></pre>
  
  <p>Oh and further down, same error here.  'Row' not defined.</p>
  
  <pre><code>Row row = sheet1.CreateRow(i);
  </code></pre>
  
  <p>Using MS Visual Studio 2013. (C#)
  Thanks.</p>