Add basic benchmarks

The intent here is to start working on a way to collect simple benchmarks on when, without really worrying about comparing performance with other libs.  It can helps us identify potential perf problems, perf regressions, and give us more meaningful data for optimizing.

This pull includes some very basic benchmarks using [benchmark.js](http://benchmarkjs.com), and runs in modern browsers and node.  In putting it together, I've already identified 2 possible areas where we can improve: time to first handler (latency, caused by falling back to setTimeout in envs where faster hacks are available), and long rejection chains (not sure yet why these perform worse than long fulfillment chains, but at least we have a way to measure).