FIX: Strict error warnings on DataExtension

PHP is throwing strict error warnings when overriding the
updateCMSFields and other functions in custom DataExtensions due to
the fact that the abstract class doesn't declare the variables should
be passed by reference
FIX: validate doesn't take var by reference

Got a bit carried away, validate doesn't take the var by reference
Merge pull request #1156 from mandrew/patch-1

FIX: Strict error warnings on DataExtension