created PropertyInterface.
added 3 more missing methods.
fixed phpdocs's reference.
BelongsTo does not depend on HasOne;
construct parameter by each of relation object.
added some comment.
an idea about many-to-many relation.
BelongsTo to use setRelation method for late-linking.
fixed using wrong name and tested it.
added isLinked method for checking state of relation.
tested the late-linking stuff.
tested late-linking for BelongsTo and fixed a bug.
no need for hasOne entry for BelongsTo.
hmm, made RelationAbstract an abstract.
renamed relationship from BelongsTo to Joined.
created Groups model and group entity.
test for Group, just copied from Basic_Tests.
tested the Groups model.
added new test, BasicCode_Test, and renamed BelongsTo to Joined.
removed unused member variables; they are in Property class.
part of JoinBy code.