讓div container能自動調整大小，以容納float children

http://stackoverflow.com/questions/16493456/parent-height-doesnt-follow-their-float-children
http://stackoverflow.com/questions/804926/make-outer-div-be-automatically-the-same-height-as-its-floating-content
http://stackoverflow.com/questions/9463069/grow-height-of-parent-div-that-contains-floating-nested-divs