Rename get_response -> build_response

It's more descriptive of what the method actually does
and it avoids confusion with the HTTP GET verb.
Remove unused is_json argument
Extract building of Fever response
Extract favicons method
Extract items and total_items methods
Extract links, unread_item_ids and saved_item_ids
Sort helper methods in order of appearance in initialize
Extract method process_params
Merge pull request #251 from Koronen/refactor-fever-api

Refactor FeverAPI