fs: add type checking to makeCallback()

This commit adds proper type checking to makeCallback(). Anything
other than undefined or a function will throw.

PR-URL: https://github.com/iojs/io.js/pull/866


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>