datacommons 0.1.0

• Initial CRAN submission with dc_get_node(), dc_get_property_values(), dc_get_property_values(), dc_get_classes(), dc_get_observations(), dc_get_resolve(), dc_get_dcids_by_wikidata_id(), dc_get_dcid_by_coordinates(), dc_get_dcids_by_name(), dc_post_sparql(), dc_has_api_key(), dc_set_api_key(), dc_set_base_url().