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.

In the first version I also set the bandwidth allocation of the torrent to high, but I have commented it out in this commit as I guess some/most? people don't want the allocation to remain high after the download has finished. This could be added in "config.ini" as an option for advanced users (high allocation to recent releases is a good way to build ratios/ul).