NZB Provider cleanup

couchpotato/core/event.py

couchpotato/core/plugins/searcher/main.py

couchpotato/core/providers/base.py

couchpotato/core/providers/nzb/ftdworld/main.py

couchpotato/core/providers/nzb/newznab/main.py

couchpotato/core/providers/nzb/nzbclub/main.py

couchpotato/core/providers/nzb/nzbindex/main.py

couchpotato/core/providers/nzb/nzbsrus/main.py

couchpotato/core/providers/nzb/nzbx/main.py

couchpotato/core/providers/nzb/omgwtfnzbs/main.py