Add vendor prefixes by postprocessing CSS with Autoprefixer

This PR changes vendor prefixes to be generated based on caniuse.com data. Browser support can be customized in `grunt-autoprefixer` settings.

> Autoprefixer interface is simple: just forget about vendor prefixes and write normal CSS according to latest W3C specs. You don’t need a special language (like Sass) or special mixins.

> Because Autoprefixer is a postprocessor for CSS, you can also use it with preprocessors, such as Sass, Stylus or LESS.

> Autoprefixer uses the most recent data from Can I Use, understands which browsers are actual and popular and adds only the necessary vendor prefixes.

> You can specify the browsers you want to target in your project.

> By default, Autoprefixer uses "> 1%, last 2 versions, ff 17, opera 12.1":
- Firefox 17 is a latest ESR.
- Opera 12.1 will be in list until Opera supports non-Blink 12.x branch.