Construct moment local to timezone

Attempts to address #11

Here I took the approach of overriding `moment.fn.local` to mutate the moment so that its representation is local to its timezone if present. So

```javascript
moment.tz("2013-01-01T00:00:00", "America/New_York").local().format()
// => "2013-01-01T00:00:00-05:00"

moment.tz("2013-01-01T00:00:00", "America/Los_Angeles").local().format()
// => "2013-01-01T00:00:00-08:00"
```

Let me know if you think this is on the right track and if so whether I should consider additional tests cases.