Add MongoCollection and adaptiveCollection abstraction to MongoAdapter.

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -1,4 +1,6 @@
 
+import MongoCollection from './MongoCollection';
+
 let mongodb = require('mongodb');
 let MongoClient = mongodb.MongoClient;
 
@@ -30,6 +32,12 @@ export class MongoStorageAdapter {
     });
   }
 
+  adaptiveCollection(name: string) {
+    return this.connect()
+      .then(() => this.database.collection(name))
+      .then(rawCollection => new MongoCollection(rawCollection));
+  }
+
   collectionExists(name: string) {
     return this.connect().then(() => {
       return this.database.listCollections({ name: name }).toArray();