CF-345: json schema

Changes to add validation to PreferencesService. Further clean up is necessary. But I would like to submit that to avoid the PR from getting too big to review. 
* Use JSON Orderly API
* Add happy path unit tests for PreferencesService API
No. This is listed out in Bryan's design note:
https://one.rackspace.com/display/cloudfeeds/Feeds+Archiving+Preferences+Service#FeedsArchivingPreferencesService-URIStructure

And it is actually the mediaType for JSON schema:
http://json-schema.org/latest/json-schema-validation.html

So I guess we should change it. OrderLy does not have its own mediaType, or at least I haven't found it googling around. I guess I should change it to application/json, then?