introduce utils::signed_to_s
use utis::signed_to_s in ttrss_api.cpp::fetch_feeds_per_category

ttrss has special categories, which are asigned ids below 0.
If using the unsigned to_s() method these result in queries to
nonexistend categories.
remove unused, wrong part of get_subscribed_urls()
ignore special categories
Merge pull request #50 from t-8ch/ttrss_api

fix stringification of category id