Make a BatchResult struct to hold batch request response

[huandu]

Just like PagingResult, BatchResult should hold all information returned by facebook batch api.

It's difficult to read data in batch request right now. Unlike other graph api response, batch api returns real result (the "body" field) with a JSON stringified string inside a JSON object. One has to parse body to JSON to use it. It's annoying. BatchResult can definitely help us to save time.

Reference: https://developers.facebook.com/docs/graph-api/making-multiple-requests/#multiple_methods

[afinkenstadt]

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