Extracted a `Aws::TreeHash` class from the GlacierChecksum plugin.

This TreeHash class is now re-usable when computing rolling tree
hashes of multiple uploaded glacier archives.

A full example has been added to the integration tests showing
how to re-use the tree hash parts from each upload part to
compute the final tree hash without re-reading the uploaded file.

See #33
Fixing the .travis.yml to correctly exclude Oj/Ox from JRuby testing.