v1.2.1

• Make aux proxy object callable. (0e63628)
• Add custom x_labels to Variable. (cbbb8f5, 7de48e0)
• Parse hex strings in ColorMixin. (148c016)
• Add sorting operators to UniqueObject. (baf45ba)
• Let Dataset inherit from TagMixin. (43395ba)
• Allow initial objects to be passed in UniqueObjectIndex init. (ebea663)
• Fix indefinite recursion in DotAccessProxy. (7bb8955)