Add some missing docs to std.traits.
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 #2201 from AndrejMitrovic/TraitsDocs

Add some missing docs to std.traits.
Merge pull request #2200 from jmdavis/datetime

Some cleanup of std.datetime.