Adds Underscore.js and Lodash.js to the benchmarks

Unfortunately, it looks like lodash consistently outperforms fast in the `.forEach()` test, and occasionally in the Array tests with `1000` items.

This *could* just be a matter of using a while loop instead of a for loop, at least on the occasional wins for lodash.

Lodash's `forEach` also uses a `while` loop, but on top of that, they don't set the context each time using `call`. They'll call the function using traditional notation unless you specify a context. I'm not sure if `fn.call()` is slower than `fn()`, but it's a possible reason for lodash's success.
probably, but that requires some poking around with the results from Benchmark.js, rather than just using `.filter('slowest')`. If you'd like to submit a PR with that... :smile: