Server: handle all uncaught errors from client connections in #client_loop
Rewrite of Message + message handling for better safety + flexibility

Rather than have a single Message class and different payloads for
certain message types, we now have an abstract Message class and several
sub-classes thereof.

Additionally, the attribute system in ObjectNode, used to inform ProxyRMI
how it should handle various methods and values, has been fleshed out with
support for class attribute support.
ObjectNode.rb: remove unneeded dependency on yaml
ObjectNode#close: clear local-references hash on connection close.
ObjectNode: fixups to properly support exported blocks in method calls
Server#handle_message: catch and return *all* errors when evaluating...

...not just syntax errors.
Object#proxy_client: new method, missed in @664d7e8

The reference returned by this object is used to determine when to refer
to an object that lives on the other end of the connection, when exporting
objects.
README.md: mention alias of `Client#fetch`
README.md: remove unnecessary hyphenation.
README.md: indicate that I can help a little with problems with ProxyRMI