JSON-RPC batch support via subrequest

[inklesspen]

Comparing empty response handling in other implementations:

• jsonrpc (C) returns either a null pointer or an empty string, depending on which layer you use.
• tinyrpc (Python) supports batch requests, but not batch responses, making the point moot
• AFJSONRPCClient (Objective-C) does not support batch requests or responses
• go/net/rpc/jsonrpc (golang) doesn't seem to support batch either
• jsonrpc4j (Java) sends an empty array, in violation of the spec
• jrpc2 (Node.js) client does not support notifications in batch requests; server appears to return an empty array, in violation of the spec
• objc-JSONRpc (Objective-C) supports only regular requests in batch mode, not notifications
• jimson (Ruby) correctly sends an empty body if there is no response, but its client assumes the body is not empty and is valid JSON. Did they test this?

[mmerickel]

@JustusW can you please look at this and decide if it meets your needs to replace #34? I think it is a better approach than the tween. It nicely uses pyramid's subrequest support that didn't exist back when the original TODO was written.

[JustusW]

This looks great! And it completely does the job, so thank you :)