title: moment.js does not convert timestamps with specific timezones to unix timestamps
body: <p>I am using moment.js to convert a bunch of timestamps in it's specific timezone to a unix timestamp like this:</p>
  
  <pre><code>var timestamp = "2015-12-29T09:35:00.000-08:00";
  console.log(moment("2015-12-29T09:35:00.000-08:00").unix();
  console.log(moment("2015-12-29T09:35:00.000-08:00").tz("America/Los_Angeles").unix();
  </code></pre>
  
  <p>The console log of both the above statements is for some reason, the same - 1451361900. This unix timestamp which it is logging is in my local timezone and not the one I asked for: "America/Los_Angeles". What am I missing?</p>