Some cleanup of std.datetime.

This is mostly some cleanup which takes advantage of some stuff like
format being pure now, but it also removes the long month name and
case-insensitive months for simple time. I don't know why it was there
(probably idiocy on my part), and it violates the format for simple time
as defined by the documentation.
Merge pull request #2200 from jmdavis/datetime

Some cleanup of std.datetime.