Relative Dates #758

Implements https://github.com/MayhemYDG/4chan-x/issues/758.

![relative dates](http://i.imgur.com/JIsO5.png)

Pretty straight port from html5chan.

* Uses `%R` in the time formatting rice.
* Times on all posts are updated when new posts are added by `Updater#update`.
* Title tooltip set to the absolute time through `Date#toString`.