Chaining setTimezone()

Hi :)

I just add `return this;` at the end of setTimezone() method.

With that, we can do :
<pre>
  var now = new time.Date().setTimezone("America/Los_Angeles");
</pre>

Thanks for you awesome module :)
Merge pull request #34 from Atinux/patch-1

Chaining setTimezone()