DBmPDO extension and DB modifications

- Added $port parameter to the __construct() in DB class
- Additional parameter for query() function in order to add ability to
use parametrised queries in extensions, DB class.
- Added the explain() method in order to save backward compatibility for
those runs where the query is parametrized but current extension knows
nothing about using parametrized queries. The algorithm is similar to
one used in PDO extension from PHP source code, DB class.
- Some improvements in DBmPDO class
- Added some comments in both files.