Closes #10: Add support for dependent plugins

* package mojo: log a warning if a plugin is declared as a dependency with RUNTIME scope
* validation mojo: include all plugins in classpath
* integration test setup mojo: copy all plugins in directory
* added dependent-plugin itest

Side work:
* extracted repeated utility code in Utils class
* use org.apache.commons.logging.impl.SimpleLog instead of creating a log4j file for validate mojo
* added fastBuild POM property