Add usage docs for moment.tz

I'm not sure what the protocol is here, but I just modified the source files. I'm assuming leaving it up to deployers to rebuild the actual html is the right way to go?

![image](https://f.cloud.github.com/assets/150329/1560647/77eada50-5010-11e3-8ba7-b14c46bfa40e.png)
Screenshot of the rendered docs.