Added optional source url restrictions to Rule

[fraserharris]

There is no easy way (that I can find) to restrict Rules to specific response URLs [1]. For example, I may want to limit a Rule to only apply to a response from a category.php page [2]. It appears the current way to do this is to be overly specific with your link extractor's xpath so it only matches links on category.php pages. This has the downside of being fragile and unprovable.

This PR:

• adds new Rule optional arguments allow_sources and deny_sources (mimicking the implementation of LinkExtractor's allow and deny)
• adds Rule.source_allowed(url) to check if url is allowed/denied
• CrawlSpider._requests_to_follow checks Rule.source_allowed for each rule
• Tests for above

[1] Example of someone else trying to solve this issue: http://stackoverflow.com/questions/22653656/how-to-make-rules-of-crawlspider-context-sensitive
[2] In my use case, I need to scrape a mix of ecommerce product pages, category pages & "super-category" pages.