use abstract for relations.
added a quick method to get id_name.
more code for HasOne and RelationAbstract.
these are defined in the RelationAbstract, so removed them.
accepts only EntityInterface (not arrays) for HasOne relationship.
BelongsTo relation (which was IsRef in wsCore. use popular name).
created a mock method, relation.
implementation of BelongsTo; uses HasOne to set it's relationship.
removed add and del method from RelationInterface and children classes.
simplify the interface.
added link for HasOne to establish relationship.
commented on setRelation.
simplified code by moving fetch/get code to RelationAbstract.
class to manage relations.
necessary for finding the right relation class (HasOne, BelongsTo, etc.)
RelationManager and relation method in EM to use it.