Use Autoprefixer to generate CSS vendor prefixes

Hi,

Here is the PR related to https://github.com/videojs/video.js/pull/824

Some documentation is definitely needed to explain the extra autoprefixer step if these PRs get pulled. There could also be a Grunt task (or something else) that would make sure that latest version of standalone Autoprefixer gets fetched from `autoprefixer-rails` repo.

Oh yeah, I also had to update `grunt-contrib-jshint` version since the current version failed to run on my machine.