Thank you for your kind answers, everyone. It seems that, a druid with barbarian archetype can definitely use (and benefit from) rage when he's using wild shape. While RAW does not clearly specify that he receives bonus damage from rage during wild shape, it seems that according to RAI the druid should receive it, since it doesn't seem logical to me that wild shape or polymorph prevents one from receiving bonus damage from rage. I really wish Paizo to specify this part in the errata someday though.
A follow-up question. Can the druid benefit from such feats like furious grab or oversized throw, after he polymorphed into a fire elemental, purple worm or sea serpent using wild shape? I mean, logically, to use the grab or throw action, the creature must have a hand. But the rulebook didn't say you have to have a hand to grab or throw. So logically, the druid can still benefit from furious grab or oversized throw feat after polymorphed? Likewise, while the description text for farabellus flip implies that this feat allows the medium armor wearer to become more agile, it didn't say you have to wear a medium armor to benefit from the feat. So the GM cannot stop me from using it even when I'm wearing a heavy armor, not wearing an armor at all, or while using wild shape?