Extracted BaseEntity::Attributes and Reference to a module
Added ValueReference < Reference type [sequel-datasets]
Added attribute initialization validation [sequel-datasets]
Validating ChildReference type [sequel-datasets]
Check BaseEntity is initialized with a Hash [sequel-datasets]
Merge branch 'sequel-datasets' of git://github.com/nandosola/sequel-uow into sequel-datasets

Conflicts:
	lib/base_entity.rb
	lib/mapper.rb
	spec/001_simple_entity_spec.rb
Merging with Nando's changes