@font-face declarations now share font-family name for different weights..

... and styles.

Deprecated font-specific variables.
Provided new means of defining font use cases and getting the font-family (with fallbacks) for a given use-case.
SASS will warn when getting a font family (via a use case) that may not have been included.
Added protection against trying to include a font-face that's not in the permitted list.
Updated readme.