first
Merge pull request #11 from Zeoh/master

part 2 finished.
Merge pull request #12 from xujiyao/master

first
zuoye
Merge pull request #13 from weijijun/master

zuoye
finished chapter3
task 2
Merge pull request #15 from Iakgun/master

task 2
Merge pull request #14 from magic8sky/master

finished chapter3
add robot
updated chap4