Update @font-face declarations, now in a separate style sheet for each variant

The format used for the @font-face declarations is now the
"Fontspring @Font-Face Syntax"[1] considered [2]
"the most simple and compatible one".

There may be no need to declare the font-weight as bold. This change is encapsulated in the last commit, which may be reverted if needed.

[1] The New Bulletproof @Font-Face Syntax
2011-02-03 (last updated: 2011-04-21)
http://www.fontspring.com/blog/the-new-bulletproof-font-face-syntax

[2] The @Font-Face Rule And Useful Web Font Tricks
2011-03-02, by Ralf Hermann
http://www.smashingmagazine.com/2011/03/02/the-font-face-rule-revisited-and-useful-tricks/