Code review/refactored version

[shukebeta]

The following issues are fixed

• Shape class has too much responsibility, while Order class is too simple
• Magic number and hard-coded strings everywhere
• Inconsistent / unclear method names
• Too many public methods, even they are only used internal
• Too much duplicate code between classes
• Using int type to save price is not wise
• If you want to define a custom ToString method for a class, it should be an override one
• No unit test to ensure the correctness of code