Add JSONP support

I worked with a different JSON-API today which made me realise that Eve doesn't have support for JSONP yet. With this feature you're able to add a `JSONP_ARGUMENT` to your config. When you request a resource you can add this argument and the response will be wrapped in a JavaScript function call. The obviously only works with JSON. If XML is requested the setting is ignored.

Example

request: `curl -i http://localhost:5000/?callback=JSONP_CALLBACK`
response: `JSONP_CALLBACK( # json response here )`