Use the stdlib's PBKDF2 implementation when available.

This is a bit faster than ours, which is good, because it lets you increase
the iteration counts.

This will be used on Python 3.4+, and, pending the acceptance of PEP466, on
newer Python 2.7s.
Typo fix
For another day
Merge pull request #2582 from alex/stdlib-pbkdf2hmac

Use the stdlib's PBKDF2 implementation when available.