utf8mb4 implementation

- Support for utf8mb4 (MySQL)
- utf8mb4-Conversation (combined with UTF-8- Conversation in ACP)
- Replaces fix in MyBB 1.6.11

TODO:
- Username validation (avoid replacements "?")
- Performance?

References:
http://community.mybb.com/thread-146488.html
http://community.mybb.com/thread-146408-post-1042011.html#pid1042011