Benchmark refactor 2

Another pass at bringing some sanity to our benchmarks.

This adds a benchmark/common.js script which can be used to run benchmarks, and has a handy thing for sicking `ab` on your http_simple.

Unused/broken benchmarks are removed, or fixed.

A `benchmark/compare.js` script can be used to make something like [this](http://static.izs.me/node-not-fast-yet.html) (Except on the terminal, not a webpage -- web output coming soon!)

Please review and share your opinions.  The goal of this is to make it pretty easy for someone to add a new benchmark.  It's a bit trickier than a test, but it should be relatively straightforward to copy one of these and then repurpose it.