Port UMLGraph to Maven

The attached patches modify the UMLGraph source code base to fit Maven's layout. They also contain a pom.xml file configured to do everything (I hope!) that build.xml was configured to do. Finally, they replace the generation of the Version.java file in favor of a properties file that gets updated by Maven when the UmlGraph.jar file is being generated. 

I would suggest to read the following

https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide

from section 7a.2 onwards and try to prepare a couple of snapshot releases with my branch. If everything works, it should be safe to merge.