add es6 await/async function

README.md

@@ -60,3 +60,15 @@ orm.connect("mysql://username:password@host/database", function (err, db) {
 });
 ```
 
+#### ES6: await/async
+```js
+await db.transactionAsync(async () => {
+	let persons = await Person.findAsync({ surname: "Doe" })
+
+	for (let person of persons) {
+		await person.removeAsync();
+	}
+});
+```
+**make sure use await in `db.transactionAsync` to catch error when function scope throw error**
+**and make sure function scope use async and await to ensure commit, or catch error throw then to rollback**

lib/plugin.js

@@ -28,6 +28,21 @@ function Plugin(db) {
 		});
 	};
 
+	db.transactionAsync = async function (cb) {
+		await db.driver.execQueryAsync("BEGIN");
+		debug("init transaction!");
+
+		try {
+			await cb();
+			debug("transaction committed!");
+			await db.driver.execQueryAsync("COMMIT");
+		} catch(err) {
+			debug("transaction rolled back!");
+			await db.driver.execQueryAsync("ROLLBACK");
+			throw err;
+		}
+	}
+
 	return {
 		define: function (Model) {
 		}