Vendor prefixes

Remove all vendor prefixes from Sass and CSS files —although CSS files still have them, but automatically added  by gulp-autoprefixer. This resolves #175. It's important to take a look at d9fd029 since it's preventing gulp-autoprefier to run properly on raw css files.

For convenience you can test which prefixes will be added with the current configuration in here: http://jsfiddle.net/3GSKZ/show/light/.

Further details in each commit.