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.
Okay, I see the new multiple-spider-per-process functionality also works with a single spider per crawler, I hadn't read up on that yet.

So as a result, all Middleware/Extension instances only ever see a single crawler + spider in their lifetime. (Do I have that right? Can I have that on (e)paper? ;)

> I don't like relying on `__del__` because it is not deterministic, it is not guranteed to be called, and it breaks garbage collector in Python 2.x

I was under the impression this is only true in the event of cyclic references (which I'm not sure if we have them here). But I guess better not to have to worry about those.

I much appreciate the explanations, so I can write better code next time.