Use a keystone for NFSN’s self-signed certificates and more

Added four keys to the keystore:

* NFSN self-signed cert
* NFSN base cert
* Starfield CA cert
* Valicert Root CA cert
update integration testing
refactor APIResponse to more closely model reality

* /account/<ID>/balance returns a JSON-parseable double
* /member/<ID>/email should return a string, but it doesn’t actually appear to work in testing with curl

Also modified tests to suit new findings.