Add new mixins

* Adjusments on document structure with comments for documentation
* Update README
* Add some mixins
    * Animation with keyframes
    * Background-size
    * Columns
    * Transform
        * Default
        * Translate
        * Skew
        * Rotate
        * Scale
        * Transform origin
    * Gradients
        * Linear horizontal
        * Linear Vertical
* Adjusment some mixins
    * Box-shadow: support for multiple parameters (shadows)