Px to em

easy-to-use SASS function - specify a size in `px`, get the size output in `em`.

Should help our gradual development away from px-based layouts.

Usage:

```scss

body {
    font-size: px-to-em(18px);
}
```

output, based on our default base font size of 12px:

```css
body {
    font-size: 1.5em;
}
```