refactor(export): expose AggregationCursor

My use case is to be able to do `foo instanceof AggregationCursor` just as one could do with `Cursor` and I don't see any reason why `AggregationCursor` should not be exposed.

index.js

@@ -13,6 +13,7 @@ connect.MongoNetworkError = core.MongoNetworkError;
 
 // Actual driver classes exported
 connect.Admin = require('./lib/admin');
+connect.AggregationCursor = require('./lib/aggregation_cursor');
 connect.MongoClient = require('./lib/mongo_client');
 connect.Db = require('./lib/db');
 connect.Collection = require('./lib/collection');