strftime and strptime support

[BurntSushi]

I thought I would have implemented these for the initial release, but I decided to cut scope so that I could get something out there.

I think it's questionable to be adding APIs like strptime. In particular, there's at least a few things to not like about it:

• It is a IMO pretty inscrutable way to parse a datetime.
• It interacts with localization in naive ways.

My retort to both of those concerns is:

• While inscrutable, it is widely used. Folks seem to have a "the devil you know" relationship with it.
• We can say up front that another crate should be used for localization purposes. But I don't think the lack of localization means we need to not do anything. There is still a wide use case for which "format datetimes as English in the Gregorian calendar" is useful.

[BurntSushi]

RFC 2822 support done on Saturday, and strftime/strptime done on Sunday. Jiff doesn't support every conversion specifier known to exist, but I think we start with a good set. And because of tighter time zone integration, we don't suffer from this problem.