title: Finding identical keys in multiple objects
body: <p>I have an object with the rough following structure. I need to find if one email exists in another object (month) and if it does, in which one. Ideally I would be able to look at all the months in which the user belongs too so I can then say something like "hello@gmail.com is active in September and October". I'm a little familiar with Underscore so I could use that.</p>
  
  <p>How can I create a new object with the email addresses as unique keys with an array of all the dates that email occurred in?</p>
  
  <pre><code>data: Object {
    2015-08-09: Object {
      hello@gmail.com: 5,
      hello2@gmail.com: 4,
      hello4@gmail.com: 6
    }
  
    2015-09-09: Object {
      hello@gmail.com: 5,
      hello2@gmail.com: 4,
      hello4@gmail.com: 6,
      hello9@gmail.com: 4
    }
  
    2015-10-09: Object {
      hello@gmail.com: 5,
      hello2@gmail.com: 4,
      hello4@gmail.com: 6,
      hello9@gmail.com: 10,
      hello5@gmail.com: 7
    }
  }
  </code></pre>