Fixes #138 - makes CleanCSS interface fully object oriented.

* use `new CleanCSS(options).minify(css)` to minify CSS.
* adds 1.x -> 2.x migration guide.
Merge pull request #169 from XhmikosR/master

Updates docs & history.
Fixes #145 - adds minifications stats when calling library with `debug: true` option.

* Refactors CLI `--debug` to use these stats too.
* Moves `originalSize` field into `stats` one.