title: Not equals in mongo mapper
body: <p>I'm trying to run a query where I want to ignore records with a certain email address...</p>
  
  <p>@foo = Bar.all(:email => 'xxx')  &lt;--- Except I want to negate where this email address exists.</p>
  
  <p>Please let me know how I can do it.</p>
  
  <p>Thanks!</p>