Introduce Spring Data Neo4j framework to the library to modify persistence layer. Now all the objects are held by Spring context and the domain model looks like in Activiti Engine (The code is not ready, tests are muted).

 Links:
 Spring Data Neo4j: http://projects.spring.io/spring-data-neo4j/
 Docs: http://docs.spring.io/spring-data/data-neo4j/docs/current/reference/htmlsingle/