M2crypto password hashing

One of the biggest things slowing down the load testing on the Raspberry Pi was the login process, which took 6.5 seconds of CPU time. By leveraging M2Crypto (which should already be installed on Pi deployments for the other crypto needs), this is brought down to 150ms (a 43x speedup). I also halved the default number of hashing iterations for student accounts, which are not high-stakes.

Could you take a quick look before I merge, @bcipolli?