Consolidate font stylesheet requests

Each 'font!google' dependency injects a new <link> element referring to
that font's stylesheet. The API supports multiple font-families in a
single request, which consolidates stylesheet requests.

This removes two HTTP requests to Google's font API.
Merge pull request #140 from mesosphere/single-font-request

Consolidate font stylesheet requests
Merge branch 'master' of https://github.com/airbnb/chronos