Font embedding support.

Adds the capabilities needed to embed fonts in SVGs.

* All `font-face` attributes are now supported. This is needed since other wise information may be missing when writing a font back to a file.
* The raw kerning information is saved in the font data to ease writing it back to a file.
* The new `WriteFont` module adds the function `makeSvgFont` to write a font (only containing a certain set of glyphs) back to SVG.
* The dependencies blaze-svg and blaze-markup are needed to create SVG output.