Use M2Crypto's PBKDF2 (if available) for password-hashing, for efficiency

Woooo 40x speedup on the Pi!
Decrease the number of password-hashing iterations for student accounts.
Fixes to address requests in code review #84.

https://github.com/learningequality/ka-lite/pull/84
Removed use_m2crypto argument as requested in pull request #84.
Cleaned up PBKDF2 code for review #84.
Merge pull request #84 from jamalex/m2crypto_password_hashing

M2Crypto password hashing