Make small changes and additions to variables

* Rearrange size variables
* Add z-index variables
* Make header-line-height unitless
Nice, Christian. I like that. And I do think the vertical spacing value should somehow relate to the line height. Right now, `0.5em` feels meaningless.

So instead of—quite verbosely—_removing_ units from the line heights, should they be unit-less by default and then we introduce this new `base-vertical-spacing` and _add_ units to that. Something like:

```scss
$base-line-height: 1.5;  // no reason to calculate off the base font size
$header-line-height: 1.25;
```

And then add in this new variable for vertical spacing:

```scss
$base-vertical-spacing: $base-line-height * 1em;
```

In the future, I would love all of these values to utilize Bourbon’s `modular-scale` function, to make everything more harmonious (sorry, sounds cheesy).