Feature/update plex notifier

Update plex notifier (port from SickBeard)

Change Plex notifications: allow authenticated library updates (port from @mmccurdy07)

Change to allow authenticated access to Media Server.
Change Config/Notifications/Plex logo, description, and label texts.

Adapted port commit/955a48b577875ac8e94ea5efb445c28c57b7d9cb from
mmccurdy07/Sick-Beard/commits/development (by Supremicus).