Set NOTIMEOUT flag in Mongo queries

By default, Mongo will close a cursor after 10 minutes of inactivity (as
in foursquare/twofishes#4). When you try to read from a closed cursor
Mongo will raise `cursor not found on server`.

However, if the indexer crashes and a cursor isn't exhausted you should
close it manually.
Merge pull request #13 from jakubsvehla/fix_mongo_find

Set NOTIMEOUT flag in Mongo queries