Support for copying folders

Client support for `copy_folder` and related tests.

https://developers.box.com/docs/#folders-copy-a-folder

```bash
running build_ext
test_finish_authenticate_error (tests.test_authentication.TestAuthenticationV1) ... ok
test_finish_authenticate_v1 (tests.test_authentication.TestAuthenticationV1) ... ok
test_refresh (tests.test_authentication.TestAuthenticationV1) ... ok
test_start_authenticate_v1 (tests.test_authentication.TestAuthenticationV1) ... ok
test_start_authenticate_v1_fail (tests.test_authentication.TestAuthenticationV1) ... ok
test_finish_authenticate_v2 (tests.test_authentication.TestAuthenticationV2) ... ok
test_handle_auth_response (tests.test_authentication.TestAuthenticationV2) ... ok
test_oauth2_token_request (tests.test_authentication.TestAuthenticationV2) ... ok
test_oauth2_token_request_error (tests.test_authentication.TestAuthenticationV2) ... ok
test_refresh (tests.test_authentication.TestAuthenticationV2) ... ok
test_refresh_v2_token (tests.test_authentication.TestAuthenticationV2) ... ok
test_start_authenticate_v2 (tests.test_authentication.TestAuthenticationV2) ... ok
test_credentials_v1 (tests.test_authentication.TestCredentials) ... ok
test_credentials_v2 (tests.test_authentication.TestCredentials) ... ok
test_add_assignment (tests.test_client.TestClient) ... ok
test_add_comment_to_comment (tests.test_client.TestClient) ... ok
test_add_comment_to_file (tests.test_client.TestClient) ... ok
test_add_task (tests.test_client.TestClient) ... ok
test_automatic_refresh (tests.test_client.TestClient) ... ok
test_change_comment (tests.test_client.TestClient) ... ok
test_change_task (tests.test_client.TestClient) ... ok
test_copy_file (tests.test_client.TestClient) ... ok
test_copy_folder (tests.test_client.TestClient) ... ok
test_create_collaboration_by_login (tests.test_client.TestClient) ... ok
test_create_collaboration_by_user_id (tests.test_client.TestClient) ... ok
test_create_folder_no_parent (tests.test_client.TestClient) ... ok
test_create_folder_with_parent (tests.test_client.TestClient) ... ok
test_delete (tests.test_client.TestClient) ... ok
test_delete_assignment (tests.test_client.TestClient) ... ok
test_delete_collaboration (tests.test_client.TestClient) ... ok
test_delete_comment (tests.test_client.TestClient) ... ok
test_delete_file (tests.test_client.TestClient) ... ok
test_delete_folder (tests.test_client.TestClient) ... ok
test_delete_no_headers (tests.test_client.TestClient) ... ok
test_delete_task (tests.test_client.TestClient) ... ok
test_delete_trashed_file (tests.test_client.TestClient) ... ok
test_download_file (tests.test_client.TestClient) ... ok
test_download_file_with_version (tests.test_client.TestClient) ... ok
test_edit_collaboration (tests.test_client.TestClient) ... ok
test_file_get_comments (tests.test_client.TestClient) ... ok
test_file_get_tasks (tests.test_client.TestClient) ... ok
test_get (tests.test_client.TestClient) ... ok
test_get_assignment_information (tests.test_client.TestClient) ... ok
test_get_client_with_retry (tests.test_client.TestClient) ... ok
test_get_collaboration (tests.test_client.TestClient) ... ok
test_get_comment_information (tests.test_client.TestClient) ... ok
test_get_events (tests.test_client.TestClient) ... ok
test_get_file_metadata (tests.test_client.TestClient) ... ok
test_get_folder (tests.test_client.TestClient) ... ok
test_get_folder_collaborations (tests.test_client.TestClient) ... ok
test_get_folder_content (tests.test_client.TestClient) ... ok
test_get_folder_iterator (tests.test_client.TestClient) ... ok
test_get_folder_iterator_zero_content (tests.test_client.TestClient) ... ok
test_get_id (tests.test_client.TestClient) ... ok
test_get_long_poll_data (tests.test_client.TestClient) ... ok
test_get_path_of_file (tests.test_client.TestClient) ... ok
test_get_task_assignments (tests.test_client.TestClient) ... ok
test_get_task_information (tests.test_client.TestClient) ... ok
test_get_thumbnail (tests.test_client.TestClient) ... ok
test_get_thumbnail_with_params (tests.test_client.TestClient) ... ok
test_get_user_info (tests.test_client.TestClient) ... ok
test_get_user_list (tests.test_client.TestClient) ... ok
test_handle_error (tests.test_client.TestClient) ... ok
test_init_with_credentials_class (tests.test_client.TestClient) ... ok
test_init_with_string (tests.test_client.TestClient) ... ok
test_long_poll_for_events_and_errors (tests.test_client.TestClient) ... ok
test_long_poll_for_events_multiple_tries (tests.test_client.TestClient) ... ok
test_long_poll_for_events_ok (tests.test_client.TestClient) ... ok
test_long_poll_for_latest_events (tests.test_client.TestClient) ... ok
test_overwrite_file (tests.test_client.TestClient) ... ok
test_post_data (tests.test_client.TestClient) ... ok
test_post_dict (tests.test_client.TestClient) ... ok
test_put_data (tests.test_client.TestClient) ... ok
test_put_dict (tests.test_client.TestClient) ... ok
test_search (tests.test_client.TestClient) ... ok
test_share_link (tests.test_client.TestClient) ... ok
test_update_assignment (tests.test_client.TestClient) ... ok
test_upload_file (tests.test_client.TestClient) ... ok
test_upload_file_with_parent_as_dict (tests.test_client.TestClient) ... ok
test_upload_file_with_timestamps (tests.test_client.TestClient) ... ok
test_matcher_eq_works (tests.test_utils.TestFileObjMatcher) ... ok
test_rollback_to_stream (tests.test_utils.TestFileObjMatcher) ... ok

----------------------------------------------------------------------
Ran 82 tests in 1.077s

OK
```