ObjectHasher implements TreeHasher (#459)
ObjectHasher implements TreeHasher (#459)

* ObjectHasher implements TreeHasher

* nits

gtank/trillian,daviddrysdale/trillian,Martin2112/trillian,gdbelvin/trillian,gdbelvin/trillian,google/trillian,Martin2112/trillian,google/trillian,daviddrysdale/trillian,gtank/trillian,AlCutter/trillian,Martin2112/trillian,RJPercival/trillian,RJPercival/trillian,codingllama/trillian,daviddrysdale/trillian,codingllama/trillian,gdbelvin/trillian,AlCutter/trillian