Added a test ensuring that Guid keys work
Replaced CreateHashKey with CreateEntityKey
Added (and fixed) failing test for cyclic association
Added DebugExtensions.DumpTrackedEntities helper method
Added failing test (and fix) for AssociatedCollection that stores the navigation property within the associated entitiy which also has a required associate
Added (and fixed) failing test: removing associate with required associate
Added (and fixed) failing test: remove associate if its required associate is not present in context
Moved some metadata stuff to Extensions