Longitudinal Benchmarks with asv

In conversation [here](https://github.com/dask/distributed/pull/278#issuecomment-221188100) it was suggested by @minrk and @ogrisel that we set up something like https://github.com/spacetelescope/asv to monitor performance of a few important computation patterns over time. This would help us to identify performance regressions as we change the scheduler.