Migrate to MySQLi Driver

Make kDBConnection class use MySQLi driver (mysqli php class) internally instead of using deprecated MySQL driver (mysql_* functions).

Plan:

1. reference to a resource (produced by mysql_connect) now becomes a mysqli class instance
2. all methods calling mysql_* function now must use corresponding mysqli class
3. test all cases, where errors are occurring (e.g. wrong sql, wrong connection data, etc.) to ensure, that all works as before
4. public interface of class isn't changed
5. check for "mysqli" instead of "mysql" php extension presence during installation