Entity#setRelation() now takes a mixed type.

This allows for the related object to be a collection or even nothing at
all.
Adding EntityCollection#getColumn().
Implementing eagerly loaded relations for has_one.
Improving docblocks in EntitySelector.
Adding has_many relation between test entities.
Implementing lazy has_many relations.
Implementing has_many for selectOne.
Adding eager loading for many to many relations.