Firefox support, better theme support, new dark theme

@ColdenCullen and I spent the weekend at [PennApps X](http://2014f.pennapps.com/) hacking on HN Special, primarily with the intention of bringing it to Firefox. Since then I have continued to bring our changes back to Chrome and improve the theme functionality. While I wouldn't call this work perfect, I think it is good enough to put into a pull request and get feedback.

If you would like to run this in Chrome or Firefox, check out the [Releases](https://github.com/PxlBuzzard/hn-special/releases) in my branch to download the extensions.

### Features

* Firefox (>34) support!
* Themes have been broken apart so that a new theme can be added in [stylus](https://learnboost.github.io/stylus/) by just modifying color variables.
* Theme loading now happens in the page head. This minimizes the number of files being loaded to one instead of all themes, as well as properly filling in the developer console with the CSS rules.

### Known Issues

* No icon in the Firefox extension list, but it is a [known issue](https://github.com/mozilla/jpm/issues/197) with jpm.
* Mark as read is broken in Firefox and Chrome.

Issues that will be closed by this PR:  #33 #36 #43 #51 #62