* Moves datastream classes from app/model/datastream into lib/artesia/datastream. This is more consistent with other gems that provide datastreams, e.g HydraPbcore, and makes for a more consistent pattern moving forward.

* Changes anything formerly named "datastreams" (plural) to "datastream" (singular), also for better consistency.

* Adds config/pbcore.yml as new way of setting configuration for hydra-pbcore gem.

* Modifies tests and Guardfile to comply with new organization / naming conventions.
Sets test db back to just 'ov_test'. Sets Gemfile for hydra-pbcore back to pointing at github.com/afred/hydra-pbcore, master branch. Updates Gemfile.lock.