Added performance benchmark.

I wanted to know what the performance impact was so I added this in. I've added my results to the README.

The `Math.` functions are likely expensive compared to the rest of the code. I was able to massively speed up [`fast-levenshtein`](http://github.com/hiddentao/fast-levenshtein) by expanding `Math.min()` into manual code. Perhaps the same can be done here. We'll never match native but I think we can get closer.