Browser compatability and minor cleanup

 * [x] Fix the cross-browser compatability of flexbox. See #15.
 * [x] Add [timeSince.js](https://github.com/reedyn/timeSince.js)
 * [x] Add date to `index.hbs`
 * [x] Clean up Sass.
   * Reorganize stylesheets.
   * Use variables in as many places as possible.
   * Rename variables.