Added backlog search capability to TvTorrents provider

- To use backlog search, username and password for TvTorrents need
  to be entered in the provider configuration menu.
- As TvTorrents remove single episode torrents and replace them
  with complete season torrents fairly quickly when a season
  finishes, you will have to do searches for complete seasons
  to get older episodes. I'm hoping to overcome this by giving
  sickbeard the ability to cherry pick specific episodes from
  season torrents in the future (at least the Transmission API
  supports this).