Add time ago tooltips to show actual date/time

This implements the feature as requested in http://feedback.gitlab.com/forums/176466-general/suggestions/4910071-gitlab-should-show-yesterday-and-actual-dates-in

Each tooltip was visually inspected to make sure it looked good in its context. Most tooltips appear above the text. In a few cases it looked better to put the tooltip below the text.

Dashboard tooltip example:
![dashboard](https://f.cloud.github.com/assets/2394772/1749261/7ee15982-6506-11e3-8150-152fa726a716.png)

Examples where the tooltip appears below the text:
Project last activity - to avoid covering the project name
![project_activity](https://f.cloud.github.com/assets/2394772/1749274/977de3c0-6506-11e3-8868-586acdd3675d.png)

Comments - the top tooltip would appear behind the previous comment
![comments](https://f.cloud.github.com/assets/2394772/1749276/ab6f18b8-6506-11e3-97da-dd1f9b3717ff.png)

Merge requests - to avoid covering merge request details "BRANCH > BRANCH" or "MERGED"
![merge_request](https://f.cloud.github.com/assets/2394772/1749277/c07d293e-6506-11e3-8bf3-74d4b1fab392.png)

Please let me know if you would like additional screenshots.