Implemented JSONRPC 2.0 batch calls as per spec

Implemented via Pyramid Tweens as suggested in TODO. All tests in test_jsonrpc.TestJSONRPCIntegration have been updated to work both batched and unbatched.

Testprotocol:
(acpdev)juw@juw:~/Projects/pyramid_rpc/pyramid_rpc/tests$ python -m unittest test_jsonrpc
...No handlers could be found for logger "pyramid_rpc.jsonrpc"
.....................................
----------------------------------------------------------------------
Ran 40 tests in 0.218s

OK
Since the spec explicitly forbids an array, we could just sent null instead, but that might lead to problems as well. AngularJS accepts an empty string as a response, so does jQuery. I do not know enough of other frameworks to guess if this could cause a problem.