Express default line-height as a unitless ratio.

Express default line-height as a ratio rather than a fixed value. Fixes #43.

Status: **Opened for visibility**
Reviewers: @jeffkamo @kpeatt

## Changes
- Adds `$leading-ratio` variable that defined the default leading for text as a unitless ratio of the font size;
- Sets the default line-height using `$leading-ratio`
- Moves the default background and type styles from `body` to `html`. If we ever switch to rem units, we’ll need to set font-size on the root node for rems to compute correctly, so moving these to html now is a good idea.