Add isEqual method to NUXDocument

This makes it easier to find a doc in an NSArray of NUXDocument. Very useful when querying an NUXDocument in an NSArray of NUXDocument, like in:

NUXDocuments *docs = ...
. . .
NUXDocument *oneDoc = ...
. . .
NSUInteger pos = [docs.entries indexOfObject:oneDoc];
. . .

This implementation of isEqual only compares UUID and changeToken.
Tthis means the comparison is not strict. For example, you could have 2 different NUXDocument referencing the same document, but the user modified some values (dc:title, ...) in one of them => isEqual would still return YES.