Add Simplified Chinese translation

Hello, I just update Simplified Chinese (zh_CN) translation.

I installed AddQuicktag from GitHub repo with zh_CN translation in my local WordPress to check the translation result. There are still some strings on the AddQuicktag setting page without translated, but I'm sure I have translated all strings from the .pot file. Those untranslated strings are German, according to Google translate.
eg:
>Wähle die jeweilige Checkbox zum Entfernen von Core Quicktags innerhalb des jeweiligen Editors zum Post Type.

Please check if there are some string not included in the .pot file. Thank you.