Use high priority when downloading recently aired episodes

- This is a feature from the NZB clients which I have found to be
  really useful. If you have a queue of backlog downloads going,
  it is really nice that when sickbeard discovers a newly aired
  episode that it is prioritized.
- Added the method "_set_torrent_priority" to the torrent
  generic client which implementations can override to support
  this functionality.
- Implemented the method in the transmission client. If the specified
  priority is high, the client moves the torrent to the top of the
  queue and sets the priority of all the files in the torrent to high.
Added option for allowing the use of high priority on recent episode downloads

- Moved detection of recent episodes from each nzb provider to "search.py"
  so its generic for both NZB and torrent providers.
- Moved priority value to the result object.
- Added option to allow high priority for recent episodes (default: true)
  to the episode search settings.
Added option to use high bandwidth allocation on high priority torrents

- Added high bandwidth allocation checkbox to torrent search settings
  (default: false)
- Option is currently only visible in the gui when using the
  transmission client.
Merge pull request #110 from dekadius/prioritizeRecentEpisodes

Use high priority when downloading recently aired episodes