cli now prints valid JSON (instead of Python dict) when no fields are sp...

...ecified.

Currently, when running this:

`python -u -m TwitterAPI.cli -e statuses/sample`

we get output like this:

```
{u'delete': {u'status': {u'id': 473954472960344065,
                         u'id_str': u'473954472960344065',
                         u'user_id': 544701609,
                         u'user_id_str': u'544701609'}}}
```

This does not appear to be valid JSON.

This PR changes the output to, e.g.:

```
{"delete": {"status": {"user_id_str": "2369620414", "user_id": 2369620414, "id": 444959584957263872, "id_str": "444959584957263872"}}}
```