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
Basically the tweens are like middleware. The exception view handler is also a tween, so if one of your json-rpc methods raises an exception you want it to be caught and processed into a response prior to your tween seeing it. Otherwise your tween will get an exception instead of a response.