Adding font preview, embed code, weight filters

Lots of new features bring this page to usable beta status, with new
CSS for new elements plus responsive adaptations. Major features still
missing:
 - More robust filtering (needs to use JS instead of CSS to handle
multiple active filters)
 - Expanded font info on selection to show available variations
(weights + italics), plus ability to select variations to include
(updated in embed code)
 - Ability to select multiple fonts
 - Button to change page font to selected font
 - Bookmarkable URL to save selections (maybe also with cookies to
restore selections?)
 - Ability to specify font subsets