Implement Star/Unstar-Feature via fever_api

Since the latest version adds a feature to the webui to star/unstar items (through the changes provided by @edgarwang ) i took a look at the fever_api and implemented these features there (mark item as saved/unsaved), because that is what is called when you star/unstar items in clients such as reeder, for example.

Also had to implement the "saved_item_ids" call. Seems to work by now, although it only delivers first 15 items due to your pagination. Did not want to mess around in that code area, mostly because i just wanted to be able to star items from mobile clients now for later reading on webui ;-) Maybe there should be implemented a "get_all" function in the future.

Some spec-tests provided, mostly copy-paste of your work i adapted, like the rest of the commit ;-)

Also noticed fever_api calls with the param "with_ids", that is not taken care of right now: 

>Use the with_ids argument with a comma-separated list of item ids to request (a maximum >of 50) specific items. (added in API version 2)