Switches from using gems "bawstun_commons" to "sufia-models", since that's what we are really looking for.
Adds initials for translating from UOIS to PBCore.
  - Uses hydra-pbcore gem.
  - Adds pbcore metadata datastream to OpenvaultAsset.
  - adds stubbed methods and tests.
Small refactor to how encoding of xml doc is specified in module Openvault::Datastream#set_xml.
Small refactor of tests re: calls to #apply_depositor_metadata.