Refs #2063. Added base accumulator so that new extension accumulators can access it and build off of it.

lib/pipeline/accumulators/index.js

@@ -1,5 +1,6 @@
 "use strict";
 module.exports = {
+	Accumulator: require("./Accumulator"),
 	AddToSet: require("./AddToSetAccumulator"),
 	Avg: require("./AvgAccumulator"),
 	First: require("./FirstAccumulator"),