Changed/removed mixins to match with the up to date specs

Changed/removed mixins to match with the up to date specs based on the "[Public service announcement: time to update your CSS3 ](http://generatedcontent.org/post/37949105556/updateyourcss3)" article and [Can I Use...](http://caniuse.com/) references (giving more importance to the first).

The idea of the new "updated" branch is to have a option to those not concern about old browser support. Probably is a restricted public, but, hey, options almost always are good! It's a lot of modifications, so it'll be a good thing if you look closely in the code based on that article.  :-)

[The Flex Box syntax rules are out to date](http://css-tricks.com/old-flexbox-and-new-flexbox/) and the `gradient` also a little bit. I not changed them because I don't know if the intention of project admits it. If yes, I'm anxiously waiting for your updates!

New year regards!