Use ld4l-foaf_rdf gem; default motivatedBy value

Changes for 0.0.2
* remove LD4L::OpenAnnotationRDF::Person class
**  replace it with ld4l-foaf_rdf gem
* remove resource_extensions.rb
**  classes inherit directly from ActiveTriples::Resource
**  remove id_prefix support from resource (expected to be part of local name minter)
* add default value for motivatedBy in comment and tag classes
* remove :3000 from default base_uri
* cleaned up tests
**  motivatedBy tests pass
**  unmodeled data tests pass when run one spec file at a time; fail with all tests running in one pass
**  update of multi-value fails - problem in ActiveTriples code