Add cache to import function

lib/sequelize.js

@@ -73,9 +73,13 @@ module.exports = (function() {
     return factory
   }
 
+  var importCache = {};
   Sequelize.prototype.import = function(path) {
-    var defineCall = require(path)
-    return defineCall(this, DataTypes)
+    if (!importCache[path]) {
+      var defineCall = require(path)
+      importCache[path] = defineCall(this, DataTypes)
+    }
+    return importCache[path];
   }
 
   Sequelize.prototype.migrate = function(options) {