fixed api usage

lib/sequelize.js

@@ -396,7 +396,7 @@ module.exports = (function() {
     Utils.tick(function() {
       self
         .getQueryInterface()
-        .startTransaction({ transaction: transaction })
+        .startTransaction(transaction, {})
         .success(function() {
           callback(transaction)
         })

lib/transaction.js

@@ -10,18 +10,18 @@ var Transaction = module.exports = function(sequelize, options) {
 util.inherits(Transaction, Utils.CustomEventEmitter)
 
 Transaction.prototype.commit = function() {
-  this
+  return this
     .sequelize
     .getQueryInterface()
-    .commitTransaction({ transaction: this })
+    .commitTransaction(this, {})
     .proxy(this)
 }
 
 
 Transaction.prototype.rollback = function() {
-  this
+  return this
     .sequelize
     .getQueryInterface()
-    .rollbackTransaction({ transaction: this })
+    .rollbackTransaction(this, {})
     .proxy(this)
 }