Refactor background-gradient methods with string-replace functions

[bcinarli]

Current background gradient method only add prefixed values. There is no fallback and W3C standart definition addition.

Use string-replace methods for defining the accepted background-gradient definition and add other fallback with one call like

@include linear-gradient(to bottom, #fff, #999);

should output

background: -moz-linear-gradient(top, #fff, #999);
background: -webkit-linear-gradient(top, #fff, #999);
background: -o-linear-gradient(top, #fff, #999);
background: -ms-linear-gradient(top, #fff, #999);
background: linear-gradient(to bottom, #fff, #999);

or

background: -moz-linear-gradient(top bottom, #fff, #999);
background: -webkit-linear-gradient(top bottom, #fff, #999);
background: -o-linear-gradient(top bottom, #fff, #999);
background: -ms-linear-gradient(top bottom, #fff, #999);
background: linear-gradient(to bottom, #fff, #999);