adding Autoprefixer

adding the grunt task autoprefixer. and all the necessary around it

pakages.json
---
these packages added:
- grunt-autoprefixer: for auto vendor prefix adding
- grunt-contrib-cssmin: because autoprefixer doesn't minify CSS (need
to check this)
- grunt-contrib-clean: remove unnecessary files (CSS files before
minify)

Gruntfile.js
---
- adding `clean` task to remove unminify files
- chainging `sass` task to support autoprefiexer
- adding `autoprefixer` task
- adding `cssmin` task to compress css file after autoprefixer task on
`dist` tasks
- adding to `watch` task autoprefixer task
- changing `grunt.registerTask` to support all needed to `autoprefixer`

scss
---
- remove the _vendor.scss file because it isn't needed any more
- _defaults.scss: doesn't use vendor mixin, because it's not needed
- style.scss: import of _vendor.scss removed due removing of the file