Update 21.0.md

在第二十一章增加一个论文的引用链接
Merge pull request #123 from Klein-Hu/patch-1

Update 21.0.md
Update 01.0.md

有强迫症的人看到注释没对齐真难受
Merge pull request #124 from tracy-e/patch-1

Update 01.0.md
Update 04.0.md

Fix some clerical error.
Merge pull request #125 from ScorpioCPH/patch-1

Update 04.0.md (Fix some clerical error)
add all chapter 01's ruby version code based on python's
Merge pull request #126 from RONGHAI/master

add all chapter 01's ruby version code based on python's
change tab to space, and small formatting change
change tab to space
Update Readme.md
Update Readme.md
replace all tabs with spaces
Merge branch 'master' of ssh://github.com/julycoding/The-Art-Of-Programming-By-July
Update Readme.md
Update Readme.md
Merge remote-tracking branch 'upstream/master'