first 3 paragraphs of the "callback" section
7 more paragraphs
3 more paragraphs
6 more paragraphs of the callback section
A few more paragraphs. The translation of section "callback" is completed
"Callback" section completed; proofread and corrected typos
Clean up two accientally left out English paragraphs
Merge pull request #25 from ericax/master

Chinese (Simplified) translation of the "callback" section