Add font logic
Save fonts in a hash to easily search for them
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.
Add font specs

Nobile font is used for these specs. It is
licensed under the "SIL OPEN FONT LICENSE Version
1.1 - 26 February 2007". See the license file
in the spec support directory.
Update README with new options