[#83] serialize json by converting xml

serializing json from db using .db.json 'format'
[#95] 400 if a parameter is not on the code list

upgrade requests to 1.2.3, should probably unpin the requirements in
setup.py,
give more details on error message,
use requests.structures.CaseInsenstiveDict to allow codes to be case
insensitive,
add participating-org.role filter,
schema now returns codelist objects for codes rather than strings,
add .db.json format for datastore backend json