Merge pull request #31 from pezy/master

Finished Chapter 6.
refactor
Merge pull request #3 from evan617/master

Thx a lot! How careless (or stupid?) I AM!
Update ex7_1.cpp

the input should include `units_sold` and `revenue`.
Create ex7_2.h
Create ex7_3.cpp
Delete ex7_2_3.cpp

`this` is unnecessary.
Rename ex7_1.cpp to ex7_01.cpp
Rename ex7_2.h to ex7_02.h
Rename ex7_3.cpp to ex7_03.cpp
Create ex7_04.h
Create ex7_05.h
Delete ex7_4_5.cpp

`this` is unnecessary. And "these functions don't change the arguments" is just one reason. The more important reason is that pointers to const objects may call only const member functions. We should use the const after the parameter list to indicate this point.
Update ex7_01.cpp

By default, copying a class object copies that object's members.
Update ex7_03.cpp

By default, copying a class object copies that object's members.
Create ex7_06.h
Create ex7_07.cpp
Delete ex7_6_7.cpp

divide the interface and the implementation.
added 5.01~5.10
Update and rename ex7_9.cpp to ex7_09.cpp

in this case, defined the `Person` parameter as a reference to const is more better.