Inconsistent diff API

The various diff methods (doc.getContentDiff, doc.getMetaDataDiff, doc.getObjectDiff, doc.getClassDif, obj.getDiff, class.getDiff...) don't treat the arguments the same way. If there are 2 arguments, some methods compute the diff between the first and the second arguments, while other do that the other way around. If there is only one argument, some methods compute the diff between "this" and the argument, while others do the other way around.

These methods should be changed so that they behave the same way:

• 2 arguments means diff from the first argument to the second argument (get differences between the first and the second versions)
• 1 argument means diff from the argument to this (get differences from the other version)