Fix use of 'fadein' function which doesn't exist in SCSS

The original `fadein(rgba(0,0,0,.1), 15%)` cannot be compiled under SCSS/SASS. It will result in the same `fadein(rgba(0,0,0,.1), 15%)` in compiled CSS stylesheets.

SCSS/SASS, however, provide a similar function `opacify` (or aliased as `fade-in`), which take the amount parameter from 0 to 1. This fix should make the compiled results correct again.

Ref: http://sass-lang.com/docs/yardoc/Sass/Script/Functions.html#fade_in-instance_method
Merge pull request #40 from zhusee2/patch-1

Fix use of 'fadein' function which doesn't exist in SCSS