Update ex9_13.cpp
Update ex9_13.cpp
Merge pull request #67 from shbling/patch-1

Update ex9_13.cpp
better format
Update ex9_15_16_17.cpp
Merge pull request #68 from shbling/patch-4

Update ex9_15_16_17.cpp
Merge remote-tracking branch 'upstream/master'
added 7.36~7.40
ex18.12.13.14
Merge remote-tracking branch 'origin/master'
Merge pull request #69 from Queequeg92/master

add solutions for ex18.12.13.14.
Merge remote-tracking branch 'upstream/master'
added 7.41~7.42
delete @Mooophy's ex7_41

1. Constructor which parameter is `istream&` delegate to `Sales_data()`
more better.
2. main.cpp cannot show the order of execution among delegating
constructors.
added 7.43~7.46