Server.rb: remove pointless use of additional thread in `run()`
Message.rb: doc-comment fixups for `Message#initialize`
Message.rb: add support for message note on `Message.invoke` class method
samples/simple-unix.rb: simplified + prettified output of this example
(Server,ObjectNode,MessagePasser) Some fixups missed in @7e5cf7a
README.md: expanded.

Added some usage-snippets, explanations, warnings, and more likely than
not at least a few embarrassing typos.
(ObjectNode, Client) move class-instantiation to ObjectNode ctor

This enables the `(...).new(klass, *klass_args)`-style instantiation
in ObjectNode, as now documented in README.md and already used in Server.
Client.rb: remove unused require, add to/re-format some comments
Server.rb: synchronize all `server_main` usages in the run mutex
samples/simple-unix.rb: changed comment to be more explanative