Create zh-cn.py

This is a specific language file for 'zh-cn'.
The former 'zh.py' should be renamed to 'zh-tw.py'.

web2py is a good project, thank you very much.
I wish I could do something for you all.
Update zh.py

fix 'langcode' from 'zh-cn' to 'zh-tw'.
Create zh-tw.py

purely a clone of 'zh.py'.
rename to 'zh-tw.py' for more precise
Merge pull request #73 from wangganggithub/master

Add language file: zh-cn.py, zh-tw.py