4.0.0.rc2 - fix issue with position mixin that cause 0 to not return null

I was having an issue where `+position(absolute, 10px 0 0 10px)` would compile to:
```css
position: absolute;
top: 10px;
right: 0;
bottom: 0;
left: 10px;
```
When I should have gotten:
```css
position: absolute;
top: 10px;
left: 10px;
```

I fixed that issue and refactored it to use a [sass 3.3 map](http://blog.sass-lang.com/posts/184094-sass-33-is-released)