title: How to check if key exists in datastore without returning the object
body: <p>I want to be able to check if a key_name for my model exists in the datastore.
  My code goes:</p>
  
  <pre><code>t=MyModel.get_by_key_name(c)
      if t==None:
          #key_name does not exist
  </code></pre>
  
  <p>I don't need the object, so is there a way (which would be faster and cost less resource) to check if the object exist without returning it? I only know the key name, not the key.</p>