Create a benchmark for easing functions

[sole]

That way we can decide whether a performance optimisation is really an optimisation or not.

[sole]

We've got this test I did: http://gist.github.com/633961

It's a start but it's still hard to benchmark because results are fluctuating quite a lot between runs of the same test in the same machine, probably due to the way JS engines work nowadays. I'm not sure if it's worth to pursue further the benchmarking thing... so I'm closing the issue for the time being.