Add Libertine font option

This depends on #22 which should be merged first.

The Libertine family of fonts is not only a free and well designed font choice, it has one of the more complete sets of Unicode glyphs around making it great for consistent typesetting across languages.  

* https://en.wikipedia.org/wiki/Linux_Libertine
* http://www.linuxlibertine.org/index.php?id=1&L=1

I saw you've previously implemented web-fonts using a combination of formats. I know this was necessary a few years back, but I'm not sure all of those are necessary now. WOFF seems to be pretty broadly supported. TTF, SVF, OTF and other formats are available for this font (and you can even generate EOT using FontSquirrel) but before I add all those to the repo I thought it would be better to review what the target browser market is right now. It looks like there are a few things in the code already that limit accessibility in older browsers. How far back do you want to target? I'm happy to update this with whatever set of resources is deemed necessary. This was just a place to start from.

I've implemented this with some special OpenType features such as ligatures and pulling in a few alternate glyphs for some characters. This gives is a touch of historic looking flair while still being a very readable serif font base with good kerning and hinting.

Also, the over-ride for Hebrew and Greek texts is kind of annoying when some of the other fonts actually have the necessary glyph support. Might I suggest that whole bit gets re-implemented with a default font preference of a mixed set, but that if a specific font is selected, it is applied across the board? The font preference widget might be able to be consolidated by an item or two if fonts are grouped by a couple of families or a client side test was done to see if some of the specialty ones were available. Having a selector widget that does nothing when you don't have the font is a little rough on users. Thoughts?