Make $limit accept 0 for all documents to be passed

`$limit`, as it is now, accepts positive integers to be passed to limit the documents handed over to the next stage.

However, to dynamically set up aggregations in programs, one might want to pass a certain amount of documents for testing, while per default all documents should be processed.

In order to be able to do it now, one needs to modify the structure of the aggregation passed in order to be able to do that. The below example is given in Go, but it is applicable for all the other languages as well, I presume.

func createAggregationNow(limit int) []bson.M {

	var agg []bson.M

	agg = []bson.M{

		bson.M{"$match": bson.M{"date": bson.M{"$lte": time.Now()}}},

		bson.M{"$sort": bson.M{"date": -1}},

	}

	if limit > 0 {

		agg = append(agg, bson.M{"$limit": limit})

	}

	remaining := []bson.M{

		bson.M{"$unwind": "$items"},

		bson.M{"$out": "foobar"},

	}

	return append(agg, remaining...)

}

Accepting 0 as a value for `$limit` which would simply pass all documents would result in cleaner code:

func createAggregationBetter(limit int) []bson.M {

	return []bson.M{

		bson.M{"$match": bson.M{"date": bson.M{"$lte": time.Now()}}},

		bson.M{"$sort": bson.M{"date": -1}},

		bson.M{"$limit": limit},

		bson.M{"$unwind": "$items"},

		bson.M{"$out": "foobar"},

	}

}

I guess a `$limit` with value 0 could actually be optimized out of the pipeline when it is parsed, but that is just a guess.