Add JSONP support

I worked with a JSON-API today which made me realised 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.

Example

request: `curl -i http://localhost:5000/?callback=JSONP_CALLBACK`
response: `JSONP_CALLBACK( # json response here )`