Modified core.models for Page and Link
Changed frontier objects (Page, Link to Request, Response)
-modified manager and tester
-removed memory.dict backend
-simplified memory.heapq and sqlalchemy backends
-modified tests
-modified domain and fingerprint middlewares to use meta
-modified settings
Fixes for scrapy frontier middleware
Clear page object content for process_spider_output()
Modified Scrapy frontier middleware for new objects.
-Added ScrapyFrontierManager (wrapper for Frontier Manager)
Added new fields to Request and Response frontier objects
Seed loaders cleanup
Modified Scrapy frontier example to use seed loader from file
Modified Scrapy frontier example to use seed loader from file
Changed frontier Components, Backend and Middleware to abc
Merge branch 'frontier-objects-as-dict' into opic-discovery
Changes to opic to use new Response/Request models