Reimplement WebClientContextFactory in tests and examples

...so that it does no certificate validation. Fastjsonrpc of course can do
cert validation (it's built around regular Agent), but it's pointless
overhead in examples and tests.
This makes the examples work with Twisted 14.0.0 while keeping backwards
compatibility with older releases. Regarding the tests, this fixes
one failure, but there is still the Reactor was unclean problem.