Relative timestamps show original on hover

This adds extra logic so that you get the original timestamp when you hover over a relative timestamp on a comment

![screen shot 2014-12-05 at 12 37 39](https://cloud.githubusercontent.com/assets/2236852/5315843/88626f54-7c7b-11e4-9969-8d64fa2628c0.png)

cc @ironsidevsquincy 

why not just set the date in the template?

also, maybe keep the 'Permalink...' text, 'Permalink to this comment (5th Dec 2014, 12:05)'