1. Removes methods and tests for Datastream::UOIS#is_video_clip?. Determined we don't need those distinctions right at that particular spot. Maybe later.
2. Adds methods and tests Datastream::UOIS#is_audio?
3. Rearranges tests for Datastream::UOIS a little for better reporting on which fixtures we are testing.
4. Adds tests for when Datastream::UOIS#is_XXXXX? should return false.
5. Adds ability to set encoding for Datastream::UOIS#set_xml
6. Updates rock and roll fixture.

Closes #90. Closes #87.