updated 'asciidoc-to-pdf-example' to use asciidoctorj-pdf and diagram's readme revision

This PR includes:

. *Updated 'asciidoc-to-pdf-example' to use asciidoctorj-pdf*: just updated the pom to use the current version of _asciidoctorj-pdf_ instead of using the old method of including the gem _asciidoctor-pdf_. The purpouse of this change is to update all examples to the latest version and usage of asciidoctorj-*.

. *Added a note about usage of `dot` command to asciidoctor-diagram-example's README*: to execute the diagram example the Graphviz command tools must be installed. This is clearly stated in the following article: http://asciidoctor.org/news/2014/02/18/plain-text-diagrams-in-asciidoctor/, but not in the example. I think it's nice to offer the information and the reference to the article here too.