Fix segfault when reading cache

When erasing elements in the vector, the iterator is reset to the
wrong element.
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
Merge pull request #45 from jspricke/master

Fix for segfault
Merge branch 'master' of git://github.com/akrennmair/newsbeuter

* 'master' of git://github.com/akrennmair/newsbeuter:
  ignore special categories
  remove unused, wrong part of get_subscribed_urls()
  use utis::signed_to_s in ttrss_api.cpp::fetch_feeds_per_category
  introduce utils::signed_to_s
  Fix segfault when reading cache