title: Underscore.js Sum of all methods of some objects
body: <p>Is there a simple way to sum object methods.</p>
  
  <pre><code>var obj = {
      a: {
          aa: 12,
          bb: 4
      },
      b: {
          aa: 5,
          bb: 100
      },
      c: {
          aa: 1,
          bb: 150
      }
  }
  </code></pre>
  
  <p>I need to sum all 'aa' together and 'bb' as well.
  Should be something like</p>
  
  <pre><code>var res = {
      aa: 18,
      bb: 254
  }
  </code></pre>