Maven multi-module refactoring

1) Added module dg-core
2) Moved all existing code into dg-core module
3) Moved all dependencies into dg-core POM, except for those that would need to be consistent across (I left guava, gson, log4j, and junit)
4) Updated javadocs to use fully qualified class names (including package) --- this is somewhat unrelated, but came through the IDE and seemed like it would be helpful given paths to dependencies might be a little more complex with multi-module structure