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.
Since scmongo was already built to support multiple spiders (self.fs[spider]), I thought it might be better to have the mongodb connection globally?
Or is the assumption wrong, and it wouldn't open a new connection per spider, if placed in `open/close_spider`?