Handle non-JSON responses from the API

Facebook API endpoints such as `/me/picture` return things other the JSON. The library made the choice to believe any non-JSON response is an error, which is not the case.

I introduced an optional fourth 'reqOptions' parameter for the api/napi method. Currently, there is only one option in there : accepting responses of content-type other than json (see the updated Readme). You can provide a content-type string, an array of content-type strings, or just the string 'any'. I added the `encoding: 'binary'` option to the request module to allow pictures, videos and such.