Issue 480, this is the implementation of the generation of the entity relationship diagrams using GraphViz, as well as an extraction of the common code the ER diagram has with GraphViz class diagrams. Includes test cases for ER Diagram generation.

git-svn-id: http://umple.googlecode.com/svn/trunk@4829 6ebf9d09-8eb1-0036-9d08-68c07d1b990a