Make sync match async

A couple weeks ago I wrote a [replacement zlib module](https://github.com/devongovett/browserify-zlib) for browserify ([now merged](https://github.com/substack/node-browserify/pull/721)) with full node parity.  The previous one, being used here, [produces different output](https://github.com/brianloveswords/zlib-browserify#test-methodology-aka-why-doesnt-the-output-match-nodes-zlib) than Node's builtin zlib module.  I figured it would be worth making the sync and async functions in this module match.  Also, should be faster! :smile: