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 13 10 56](https://cloud.githubusercontent.com/assets/2236852/5316173/2e477b36-7c80-11e4-8b77-0997bfffd66e.png)

Also adding times to comments over a week old:
![screen shot 2014-12-05 at 15 11 31](https://cloud.githubusercontent.com/assets/2236852/5317701/071cd432-7c91-11e4-9f5c-fe2f999776cf.png)

Relative timestamps are now 24h to match our global timestamp formatting.

cc @ironsidevsquincy