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.
@kmike, sorry, I should explain. The class is opening the mongodb connection on instancing, while open_spider is then opening an individual "database" per spider. It's always been this way here (and I thought it was a nice way of doing it), except I added the destructor for the benefit of MongoReplicaSetClient which [wants to be closed cleanly](https://api.mongodb.org/python/current/api/pymongo/mongo_replica_set_client.html#pymongo.mongo_replica_set_client.MongoReplicaSetClient).

But since you're much deeper into python's internals, if you believe the destructor can be problematic in this case, we'll just have to change it?