Use a sliding window for rho-stats for large I.

I was going to use a fancy huristic based on-line median estimator
here, but a 1024 entry moving median should be sufficiently robust and
not too slow.  If it turns out to suck in practice, P^2 or something
still is an option, but this is a lot easier to code.
Add a note about Go 1.4 and go.crypto. (No functional changes).