GSoC Per-spider settings

Per-spider settings task of the 2014 GSoC, implemented on top of the #816 pull request.

This changes are based on the [SEP#19](https://github.com/scrapy/scrapy/blob/f3f3a440d3cf6c8710b65a954f9e8fd23391b2b2/sep/sep-019.rst).
It seems that CrawlerRunner.spiders is not documented. I'm not sure I like that CrawlerSpider knows about SpiderManager (and it can be used without it, as shown in your new example).