dgram: changes Socket.close() to return itself

This commit adds a return statement to the dgram.Socket.close()
function that returns itself after it finishes. This follows along
the functionality of the more popular and, dare I say, father-library
`lib/net.js`.

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


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