This commit adds support for connecting via the websocket transport
protocol on both, client and server parts.
In addition, the standalone server implemention is more complete, allowing
to manage policies and server ports via command line arguments as well
as adding a command line client tool for subscribing to a broker and/or
publishing messages to it.
When running all tests in a row, the testRetain() test would
pollute subsequent tests by leaving a retained message on the
server. Fixed by extending to testRetain() test also test
clearing all retained messages at last.
