dgram: make 'close' event async

Emit the close event asynchronously, after the close, as it is with the
net/http close events.

PR-URL: https://github.com/iojs/io.js/pull/217
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
dgram: close() should accept a callback

Like net, http, and https server.close, and socket.end(), etc.

PR-URL: https://github.com/iojs/io.js/pull/217
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
doc: add tls server.close() callback docs

Also, tests to confirm its existence.

PR-URL: https://github.com/iojs/io.js/pull/217
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>