Fix TCPServer duck typing

Ruby's TCPServer takes 1 or 2 arguments.  This makes
Celluloid::IO::TCPServer operate the same, so that it can be a drop-in
replacement in cases where the Ruby TCPServer's hostname is not
specified.  Fixes #67.
Merge pull request #68 from knewter/feature/fix-tcpserver-ducktyping

Fix TCPServer duck typing