JCLOUDS-457: Added TreeHash util class and its test.

The TreeHash class stores two HashCodes, the linear and the
tree HashCode. The TreeHash class also has an static inner class
named Hasher, which allows us to calculate the hashes.

Both Hashes (tree and linear) are needed for the upload archive
and the multipart upload operations. When completing a multipart
upload operation we also need to calculate the tree hash of the
individual part's hashes.