Fixed Fever API implementation

* Command "saved_item_ids" must return a CSV list of ids (not a JSON array)
* Command "unread_item_ids" must return a CSV list of ids (not a JSON array)
* Reeder app needs "since_id" support for the "items" command
* Implemented "mark" command for "item" ("read" and "unread")
Merge pull request #158 from stefanoverna/fever-reeder

Fixed Fever API implementation (fix #156)