Started impl. type converters for Date, Time and DateTime #305

This will be called before saving to the database (Neo4j::ActiveNode::Persistance) and reading properties from the database (Neo4j::ActiveNode::Initialize).

It should use the type value from the active_attr. gem
E.g. Person._attribute_type(:born) #=> DateTime
Each property has to be checked before reading and writing to Neo4j and translated if of type Date, DateTime or Time