Add some missing docs to std.traits.
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.
Converted more of std.datetime's examples to ddoc-ed unit tests.

The ones at the end which are removed from the docs are removed because
they're private, so there's no point in having an example in their
documentation.