Improved Pneumatic's handling of web fonts

Web fonts are now only linked to if they are being utilized.

- Added the GOOGLE_FONTS setting to Pneumatic, which is used to
automatically generate the <link> tag needed to import the fonts in the
list (and updated theme README accordingly).
- Font Awesome is only imported if the SOCIAL_ICONS setting is present.
- Changed all URLs to be protocol-independent.

Generated by commit f87e2cff9df1794f8d58676e089d91ee3496bab2; pushed by Travis build 95.