title: How to implement authentication?
body: <p>I'm relatively new to Opa, but I've been keeping my eye on it for a while now. And I really like where it's going.</p>
  
  <p>One thing that I can't get my head around is authentication in Opa.</p>
  
  <ol>
  <li>How do I track if the current user is logged in/has specific privileges?</li>
  <li>Are there any built-in validators for common stuff like email, etc.?
  ... and overall, what is the recommended way to build authentication with Opa?</li>
  </ol>
  
  <p>I guess my bottom line question is: How would I approach to build an authentication like <a href="http://jetleague.com/" rel="nofollow">http://jetleague.com/</a> does?</p>
  
  <p>Thanks,</p>
  
  <p>-- Arthur</p>