練習物件導向 (Object-Oriented)
class內的function叫methods、instances 就像class的複製品，但可以獨立被改變
當創立一個新的 instances 用__init__ 初始化、善用 inheritance (繼承)，節省時間
Python init and self what do they do?
http://stackoverflow.com/questions/625083/python-init-and-self-what-do-they-do
Why do we use init in python classes?
http://stackoverflow.com/questions/8609153/why-do-we-use-init-in-python-classes
大致筆記重要的部份：
