Add JSON format for Batch

Our batch request and response format today is multipart/mime. We've had a lot of pushback from the beginning that this format is difficult to construct and parse; there are no off-the shelf tools or libraries for doing so.

Now that we have a JSON format for CSDL, a JSON client no longer needs an XML parser to read CSDL, but they still have no way of expressing or consuming a batch without parsing the difficult to understand/complex multi-part mime.