Add fonts and update formatting prawn converter


- Color default is empty string == not set value
- write_text is the _only_ method that writes text
  to prawn. It will do any additional formatting
  that applies to the entire text section.
- inline_formatting_for -> inline_format
  method does better inline formatting based on
  given style.
Set prawn version to use

The 2011 0.12.0 version works less well with
certain options.