Import/export modulestore-stored asset metadata as XML.

Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML from exported course into modulestore.
Minor changes to asset XML schema definition.
@cpennington The backtrace demonstrating why this is necessary is below:
...
File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/lms/djangoapps/courseware/management/commands/export_course.py", line 36, in handle
    export_course_to_tarfile(course_key, filename)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/lms/djangoapps/courseware/management/commands/export_course.py", line 72, in export_course_to_tarfile
    course_dir = export_course_to_directory(course_key, tmp_dir)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/lms/djangoapps/courseware/management/commands/export_course.py", line 92, in export_course_to_directory
    export_to_xml(store, None, course.id, root_dir, course_dir)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/common/lib/xmodule/xmodule/modulestore/xml_exporter.py", line 67, in export_to_xml
    course_assets = modulestore.get_all_asset_metadata(course_key, None)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/common/lib/xmodule/xmodule/modulestore/mixed.py", line 87, in inner
    retval = func(field_decorator=strip_key_collection, *args, **kwargs)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/common/lib/xmodule/xmodule/modulestore/mixed.py", line 365, in get_all_asset_metadata
    return store.get_all_asset_metadata(course_key, asset_type, start, maxresults, sort, **kwargs)
  File "/home/jenkins/workspace/edx-all-tests-auto-pr/SHARD/1/TEST_SUITE/unit/common/lib/xmodule/xmodule/modulestore/xml.py", line 869, in get_all_asset_metadata
    raise NotImplementedError()