Add "for_json" method to support simplejson serialization.

Hi,

I have added a `for_json` method to the `arrow.Arrow` class then serialize the `Arrow` instances inside a dictionary will be possible.

``` python
>>> import arrow
>>> import simplejson as json
>>> json.dumps({'start': arrow.get(0)}, for_json=True)
'{"start": "1970-01-01T00:00:00+00:00"}'
```

Please review it. Thanks.