add a tutorial like flappy bird.
Merge commit 'dd747e83242f1cc1d17d0852eb815cebc931909c'
modify file name.
add articles of ‘about cocos2d-x’.
add articles about ‘cocos2d-x basic concepts’.
Merge pull request #129 from Ivenyang/master

add article and tutorials.
Merge pull request #130 from u0u0/master

add sprite-tutorial & menu-tutorial
fixed #3958, Tutorial about "how to use TexturePacker to optimize the spritesheet"
fixed #4105, Tutorial about "how to create buttons in cocos2d simple radio and toggle"
issue #3966, Tower defense game preface.
issue #3966, Tower defense game chapter1.