store importCache in sequelize instance

lib/sequelize.js

@@ -41,6 +41,8 @@ module.exports = (function() {
 
     this.daoFactoryManager = new DAOFactoryManager(this)
     this.connectorManager  = new ConnectorManager(this, this.config)
+
+    this.importCache = {}
   }
 
   Sequelize.Utils = Utils
@@ -73,13 +75,13 @@ module.exports = (function() {
     return factory
   }
 
-  var importCache = {};
   Sequelize.prototype.import = function(path) {
-    if (!importCache[path]) {
+    if (!this.importCache[path]) {
       var defineCall = require(path)
-      importCache[path] = defineCall(this, DataTypes)
+      this.importCache[path] = defineCall(this, DataTypes)
     }
-    return importCache[path];
+
+    return this.importCache[path]
   }
 
   Sequelize.prototype.migrate = function(options) {