Allow embedding of custom fonts

The current pdf plugin does not allow to embed custom fonts in the pdf output. All fonts are hardcoded in the xslt stylesheet fo-styles.xslt.

I think it only takes two steps to fix this:

• allow the user to specify a custom userconfig.xml file, where he can register his custom fonts
• replace all hardcoded font references in fo-styles.xslt by the value of a new property that defaults to the current basefont.