added ex7_16 ~ ex7_22
changed ex7_23_24_25.cpp to ex7_23.h/cpp and ex7_24.h

1. `const pos h`'s `const` is unnecessary.
2. the second contructs get contents by a `while` loop. unnecessary.
3. the third contructs may not get the point of question. (**a character**)
4. we should add inline member functions in this exercise.
Update README.md of Ch7

improve formating
separate the header and cpp.
added ex7.26
Create .gitignore
Create LICENSE
fixed some bugs and separate some codes
added Syncing for README.md
Merge remote-tracking branch 'upstream/master'
fix the exercise 5.4
Merge pull request #8 from evan617/master

fix the exercise 5.4
merge with origin
changed ex7_31.cpp to ex7_31.h

1. remove redundancy of include.
2. remove `class X`
fixed bugs
rewrite ex7_32

1. The question said: "clear is ... a friend of Screen", not the entire
class.
2. we should put the two class in a single header.
Update ex5_17.cpp

Before it will always output the"ture"
Merge branch 'gupenghu-patch-1'
fixed ex5_17
Merge remote-tracking branch 'upstream/master'