Add EventMachine.attach_server

This adds the `EventMachine.attach_server` & `EventMachine.attach_socket_server` methods for registering an existing FD/socket created outside of EventMachine. It's a simplified version of #271, but without the CLOEXEC/autoclose stuff. This allows sockets to be shared between parent & child processes like with unicorn's fork-then-exec graceful restart functionality.