Add 'json_data' option to force data JSONification

```
my_api = tortilla.wrap('http://my.api.com/', json_data=True)
```
will make sure that when `data` is sent to the API, it is always JSONify before being sent to `request`.
It also adds the corresponding header `{'Content-Type': 'application/json'}`.

This kind of options could be very useful for JSON-only APIs.
This should probably tell
```
if data is not None and self.json_data:
```

Sorry about that...