Add Maven plugin for API Documentation reporting

* Create a jasdoc-parent POM with 2 modules (jasdoc-core
  and jasdoc-maven-plugin) to separate the JaSDoc core
  components and the command line utility from the maven
  plugin.
* Use SLF4J instead of Apache commons-logging, to make
  sure that logging within the Maven plugin works as
  supposed to. SLF4j WILL become the logging facade in
  Maven 3.1.0 and is by now the de-facto standard in
  a modern Java world, so using SLF4J seems like quite
  a good choice.