Float and double precision, basic repositories, entity tests

Fix 5 - Float and double values as domain object signature - mostly used in ValueObjects
Fix 4 - Basic model for repository pattern
Fix 2 - Test example of implementation of entity to show that two entities are equal if they have same identifier and the same class/type hierarchy
Fix 1 - Test example of implementation of IDomainObjectSignature for Entity class

Change of namespace, from Zed to Zed.Core.