Update translation
- all 'invoice's are changed to bill, since there is no real equivalence in Simplified Chinese.
Invoice is issued before payment, but Chinese 'invoice/FaPiao' is a state printed RECEIPT issued only after payment. Regarding the function of the invoice module, 'Bill/ZhangDan' in Chinese seems by far best understood by most ppl.
Merge pull request #989 from zcattacz/zh_CN

Update zh_CN translation