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
It is an interesting approach to handle the "common-case" (a non-batch request) here in an `except TypeError` block. It's elegant but also gives me an uneasy feeling. That's all I have to say about it right now.