Activity stream should show timestamp on mouseover

Most instances of activity streams that have a vague date (like 1 hour ago, 2 minutes ago), you see on the exact timestamp when you mouseover (for instance Github or Twitter). Since we have the data anyway, we should show this to the user.

Example: http://littlebigdetails.com/post/35910906518/github-hovering-over-relative-timestamps-shows-a