Option to prefer single episode releases over season releases:

- Added option "prefer_episode_releases" to config.ini. If a complete season of a show is added to the backlog this option will make sickbeard prefer single episode releases for that season over a comlete season release.

Backport of https://github.com/johnronk/Sick-Beard/commit/1ec7c61023d89911b0ebf1c3c9a59d848aca3c68 into dev branch
Merge pull request #348 from WebSpider/dev-prefer_episode_dl

Add option to prefer episode downloading