Expand human_time_diff() from minutes/hours/days to also include weeks/months/years. Fix off-by-one issue.

props SergeyBiryukov, westi.
fixes #9272.



git-svn-id: http://core.svn.wordpress.org/trunk@24582 1a063a9b-81f0-0310-95a4-ce76da25c4cd