appengine_xblock_runtime 能在 XBlock 环境运行的开发包，尤其适用于在访问 Google App Engine 数据库时。
在 Google App Engine 应用程序的时候需注意以下几点：
Create a lib/ folder in your application.
Download the XBlock repo from GitHub into your lib/ folder.
In the lib/XBlock folder, execute: python setup.py egg_info
Download this runtime package into your lib/ folder.
In the lib/appengine_xblock_runtime folder, execute: python setup.py egg_info
Register the two packages by including in your application code:
