Review "Internationalized String Preparation"

see 

1. Preparation of Internationalized Strings ("stringprep") => https://tools.ietf.org/html/rfc3454

2. Lightweight Directory Access Protocol (LDAP):  Internationalized String Preparation https://tools.ietf.org/rfc/rfc4518.txt

3. unicode 'internationalization' FAQ => http://unicode.org/faq/internationalization.html

