dgram: change Socket.bind() to return itself

This commit changes `lib/dgram.js` Sockets to, when
they are bound to a port / IP, return themselves. This
is done in order to allow chaining of methods and be
in accordance with the `lib/net.js` library.

PR-URL: https://github.com/iojs/io.js/pull/214


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>