Add json parameter

Closes #2025
Sorry I'm late to the party, was out - wouldn't this also change the meaning for anyone who creates a Request by hand, eg

```python
params = {'date_created': '1-1-14'}
req = Request('GET', 'http://jsonip.com', {'User-Agent': 'foobar'}, None, {}, params)
```

where now suddenly params is JSON data?