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).
@afawcett The comment by @capeterson was on the previous pull request where Github had managed to mangle the diff and contents of the request. When I get a chance I'll update this pull request with a revised version which the changes suggested so you can merge it in.