proposed deprecation and merge of  scrapinghub/scmongo

I believe this project might escape some bitrot by inclusion into scrapylib.

Since there are so many different scrapy+mongodb projects around now, I thought a rename might be in order but feel free to ignore/change the last commit.
Cache storage is created by HttpCacheMiddleware downloader middleware; downloader middlewares are instantiated by Downloader (using DownloaderMiddlewareManager); Downloader is created by ExecutionEngine; ExecutionEngine is created by Crawler. Crawler is instantiated from spidercls - it only supports a single spider. This means that if users want to use multiple spiders in a single process they instantiate multiple Crawlers, and as a result they have multiple cache storage instances; this means connection is not shared even if it is created in `__init__`. 

It is a good thing because spiders don't necessarily share options, so it is possible for them to use different databases.

I don't like relying on `__del__` because it is not deterministic, it is not guranteed to be called, and it breaks garbage collector (deallocation based on refcounts is still ok).

`__del__` could help if Scrapy doesn't call `close_spider` for some reason, but that would be a bug in Scrapy.