JSON-RPC2 compliant errors

Big changes but jsonrpc2 compliant and resolve #2 . Http status code are not json-rpc2 compliant. The main reason is that error are  rpc related and not request related. With jsonrpc2 you can have batch rpc (I plan to implement this also). Anyway, we don't need to judge if json-rpc2 is good or not, we only have to implement specifications as they are.

Can you review this code ?