A ConcurrentModificationException is raised when removing an
item from projectAssignments during its iteration. In order to
avoid this, a copy of the collection is made to iterate over it.

This commit does these things:
	- Adds Projects and ProjectsTest for basic operations with Project.
	- Ensures that delete operations preserves the database integrity (i.e.
	  no FK constraint exceptions). We do this in several ways:
		- By adding CascadeType.REMOVE in @OneToMany where appropriate (e.g: Projects->ProjectAssignment)
		- By manually setting to NULL orphan entities (e.g: Department->Employee)
		- By manually removing orphan entities when no @OneToMany is available
		  (e.g: Employee->ProjectAssignment)
	- Adds CascadeType.PERSIST in Project->ProjectAssignment to automatically persist ProjectAssignments
	  that were added to a given Project.
	- Refactors tests to avoid repetitive JDBC code that inserts some registers before the test.

With this version:
	- There is no need to declare your entities in <class> elements in persistence.xml.
	- You can open and close an entitymanagerfactory in each test class without hangs
          between test classes.

It also moves transaction management out of Departments.

Creates a Departments class with basic
Department management with the EntityManager.