Allow custom transports to be passed to Python's XML-RPC library.

Custom transports have the ability to modify the network behavior of the HTTP requests, to do things like proxying (see #41), timeouts (see #52), SSL certification validation (see #60), and more.
Fixes #61.
Add documentation for new `transport` parameter to `Client`.
Add changelog details for 2.2 release.
Add changelog details for 2.3 release.
Version 2.3.