strptime/strftime: remove %y support.

This has been removed since the POSIX standard
requires guessing the century from a %y strptime,
and also states that the method of guessing may
change in future standards. This is too unstable
to use.
Merge pull request #31 from benfitzpatrick/strptime.remove-percent-y-token

strptime/strftime: remove %y support.