Add gradients and organize in files

Add gradients support:

```sass
/* Examples */
background linear-gradient(left, #f00, #f80, #ff0, #0f0, #00f, #60f, #c0f)
background-image linear-gradient(180deg, #f0f, #f66)
background linear-gradient(45deg, #66f 10%, #f80 30%, #ffc 60%)
```

Also separated all mixins in files to easily mantain them.