net,dgram: return this from ref and unref methods

Modifies the following methods to return the instance instead
of undefined, to allow for chaining these methods:

- net.Server.ref
- net.Server.unref
- net.Socket.ref
- net.Socket.unref
- dgram.Socket.ref
- dgram.Socket.unref

PR-URL: https://github.com/nodejs/io.js/pull/1768


Reviewed-By: Evan Lucas <evanlucas@me.com>