Automated performance testing

[tarcieri]

It looks like performance regressed rather severely in a commit reverted here:

#87

It would be nice to detect such regressions automatically and prevent them in the future

[vaibhavsingh1993]

Hi,
I can help out with a performance tool for the JavaScript implementation if you are not working on it already.

[tarcieri]

@vaibhavsingh1993 that'd be great! In particular I'd be curious about in-browser performance, possibly using something like PhantomJS?

On the Node side I'd like to support a native Rust backend using Neon, so I'm less concerned about the performance of the existing code on Node.