title: check if function is a generator
body: <p>I played with generators in Nodejs v0.11.2 and I'm wondering
  how I can check that argument to my function is generator function.</p>
  
  <p>I found this way <code>typeof f === 'function' &amp;&amp; Object.getPrototypeOf(f) !== Object.getPrototypeOf(Function)</code> but I'm not sure if this is good (and working in future) way.</p>
  
  <p>What is your opinion about this issue?</p>