Applied "Custom headers can be sent with request."

Commit 3dd69f9ae38c9b88c4bf84d4b67e60054c567cdc from https://github.com/dejw/jsonrpclib
Added tests for the additional headers

Applied commits from https://github.com/dejw/jsonrpclib:
- 53d38c3f4cf853e241e442d696211fa54828c3ab: Added tests for additional request headers feature.
- 9e63239d010e8bfb8c752a865b5c893c304400c3: Added test for notifications.
- 0a3bbe68cebfaac72d0f799a3854627b5cea8468: Added test for nesting _additional_headers context.

Tests have been modified to handle Python 2 / Python 3 compatibility
Updated README file