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
I thought about this for some time, the problem is, there is really only one way to check if the body is json decodable, and that is by trying it. Technically we could check if it is a list instead of catching TypeError, on the other hand, if anything happens I wouldn't want to implement my own error handling and instead I'd like to rely on the default handling.

All said, try/except is often considered to be the "pythonic" way, and in this instance I couldn't find anything that speaks against it.