Use autoprefixer to add vendor prefixes

[DaveKeehl]

Currently vendor prefixes are added programmatically to every CSS property. A better solution would be to install and integrate autoprefixer, so that this task is done automatically.

Some resources:

• https://github.com/postcss/autoprefixer#javascript
• https://raw.githubusercontent.com/ai/autoprefixer-rails/master/vendor/autoprefixer.js
• https://github.com/autoprefixer/autoprefixer.github.io/blob/master/src/main.js