Recreate dynamo db instance on .local()

The current solution requires .local() to be called first in order to
use dynamo db locally.

Recreating dynamo db instance when calling .local() solves issues like
script import order.

lib/index.js

@@ -3,9 +3,16 @@
 var Schema = require('./Schema');
 var Model = require('./Model');
 var https = require('https');
+var AWS = require('aws-sdk');
 
 var debug = require('debug')('dynamoose');
 
+function createLocalDb(endpointURL) {
+  return new AWS.DynamoDB({
+    endpoint: new AWS.Endpoint(endpointURL)
+  });
+}
+
 function Dynamoose () {
   this.models = {};
 
@@ -50,10 +57,15 @@ Dynamoose.prototype.model = function(name, schema, options) {
 
 Dynamoose.prototype.VirtualType = require('./VirtualType');
 
-Dynamoose.prototype.AWS = require('aws-sdk');
+Dynamoose.prototype.AWS = AWS;
+
+Dynamoose.prototype.driver = function driver(driver) {
+  this.dynamoDB = driver;
+};
 
 Dynamoose.prototype.local = function (url) {
   this.endpointURL = url || 'http://localhost:8000';
+  this.driver(createLocalDb(url));
   debug('Setting DynamoDB to local (%s)', this.endpointURL);
 };
 
@@ -80,9 +92,7 @@ Dynamoose.prototype.ddb = function () {
   }
   if(this.endpointURL) {
     debug('Setting DynamoDB to %s', this.endpointURL);
-    this.dynamoDB = new this.AWS.DynamoDB({
-      endpoint: new this.AWS.Endpoint(this.endpointURL)
-    });
+    this.driver(createLocalDb(url));
   } else {
     debug('Getting default DynamoDB');
     this.dynamoDB = new this.AWS.DynamoDB({