linear/radial-gradient-parsers rewrite

Removed the `linear-gradient` and `radial-gradient` functions to prevent stepping on standard CSS functions.
Rewrote the `background(...)` and `background-image(...)` to parse gradients.

Mixins will still works the same:

```scss
@include background( url("a.png"), linear-gradient(red, green), center no-repeat orange scroll );

@include background-image(linear-gradient(to top left, orange, red));
@include background-image(radial-gradient(at 40% 60%, orange, red));
```