Accept "host:port" notation for MySQLi

As announced in #31 here is a backward compatible patch for the MySQLi class which now accepts "host:port" notation.

Benefits:
- now possible to specify a custom port (was impossible prior to this)
- MySQL and MySQLi classes should now be 100% interchangeable without having to modify config file in end user consumer script, which is probably a good thing given that `mysql_` functions are [deprecated](http://php.net/manual/en/changelog.mysql.php)

The patch features a new function in ezSQL_core (function `get_host_port()` to get host and port from a "host:port" string) if anyone fancies patching ezSQL_sqlsrv 

I wanted to make the same kind of patch for the PDO class but the constructor function is totally different and I would have ended up with a non backward compatible function, so I dropped the idea.