Update fflib_SObjectSelector.cls getOrderBy() default behaviour

Changed the default value of getOrderBy() as not all objects have a 'Name' field (e.g. OpportunityContactRole, CampaignMember).
Again, not sure what @capeterson thoughts would be on this, but i would probably have just inlined the property getter code above. But curious as to what are your motivations for using this private property accessor convention here and below?