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
As I said in the PR comments, I think we want to add `above=EXCVIEW` here to be explicit about the ordering.