垂直居中方法
Seven Ways of Centering With CSS
http://demosthenes.info/blog/723/Seven-Ways-of-Centering-With-CSS
Absolute Centering in CSS
http://codepen.io/shshaw/full/gEiDt
http://demosthenes.info/blog/723/Seven-Ways-of-Centering-With-CSS
http://codepen.io/shshaw/full/gEiDt