add pg_list_sets function per #10
merge master into pg_list_sets
workaround to oaih_list_sets bug(?) so pg_list_sets(transform = FALSE) actually returns xml. #10
merge master into pg_list_sets
add XML package to DESCRIPTION
fix email address in description
Merge pull request #36 from ropensci/pg_list_sets

List set structure of pangaea database