Added aggregator pom and plugin version update tool

This PR includes the points discused in #2 and the updated asciidoctor-maven-plugin to fix #1.
The aggregator includes the examples as sub-modules with no dependencies from them, this preserves the independence of each example, but still allows to build all of them together.

About the tool to update the versions, the `versions:update-properties` option does to work since it validates the versions against maven central. This makes impossible to test locally installed versions.
Instead, gmaven-plugin have been used to execute a groovy script that updates the poms copying certain properties from the aggregator (I missed gradle here).

BTW: I've seen that images are not included in the pdf's examples, but since a new integration with pdf is on the way I'll just check it with the other tool.