title: PyCharm warns for unresolved reference builtin datetime module
body: <p>I just installed the latest version of PyCharm (4.5).</p>
  
  <p>Now I am experiencing unresolved reference errors.
  On the top of my code I have:</p>
  
  <pre><code>from datetime import datetime
  </code></pre>
  
  <p>OS is Ubuntu 15.04.
  Already did the Invalidate Cache/Restart several times. No difference.
  The Project interpreter of my project is set to Python 2.7.6.
  Already reloaded the Interperter Paths.</p>
  
  <p>Code works fine, it's just the IDE that produces an annoying error and no more autocomplete.</p>