Support custom jsonSerializer

@davidcheung đang làm issue này rồi.

Từ ngày 12/5/2016.

See https://github.com/strongloop/loopback/issues/1650#issuecomment-161920555

> ``` js
> // a custom JSON serializer function for producing JSON response bodies
> // @param sanitizedData: response data containing only safe properties
> // @param originalError: the original Error object
> jsonSerializer: function(sanitizedData, originalError) {
> if (originalError.name === 'ValidationError') {
> var details = sanitizedData.details || {};
> sanitizedData.issueCount = details.codes && Object.keys(details.codes).length;
> }
> return sanitizedData;
> }
> ```