Add methods to insert and delete references

Content owners can now create references pointing to existing claims with

    content_owner.create_reference claim_id: 'CLAIM123', content_type: 'audiovisual'

References can also be "soft" deleted with:

    reference = Yt::Reference.new id: 'REF123', auth: content_owner
    reference.delete

This "soft" deletion sets the status to "inactive" (YouTube does not provide a
method for really deleting a Reference).