Implement issue# 9337: There's no Duration.max.

Added Duration.max and Duration.min.
Added TickDuration.max and TickDuration.min.
Some tweaks to Duration's documentation.

It erroneously mentioned addYears and addMonths as functions on
std.datetime.Date.
Merge pull request #387 from jmdavis/time

Implement issue# 9337: There's no Duration.max.