Reveal annotation actions when cursor hovers over card

@dwhly asked me to revert the card behaviour back to having the annotation actions revealed on hover. So here I present the changes and a dokku for discussion.

Dokku: https://hide-card-actions.dokku.hypothes.is

![screen shot 2014-09-24 at 11 53 06](https://cloud.githubusercontent.com/assets/47144/4386441/99c53a58-43d0-11e4-963b-3fd028e9794d.png)