Add new MongoSchemaCollection class that manages schemas for all collections.

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -1,9 +1,12 @@
 
 import MongoCollection from './MongoCollection';
+import MongoSchemaCollection from './MongoSchemaCollection';
 
 let mongodb = require('mongodb');
 let MongoClient = mongodb.MongoClient;
 
+const MongoSchemaCollectionName = '_SCHEMA';
+
 export class MongoStorageAdapter {
   // Private
   _uri: string;
@@ -38,6 +41,12 @@ export class MongoStorageAdapter {
       .then(rawCollection => new MongoCollection(rawCollection));
   }
 
+  schemaCollection(collectionPrefix: string) {
+    return this.connect()
+      .then(() => this.adaptiveCollection(collectionPrefix + MongoSchemaCollectionName))
+      .then(collection => new MongoSchemaCollection(collection));
+  }
+
   collectionExists(name: string) {
     return this.connect().then(() => {
       return this.database.listCollections({ name: name }).toArray();