Changed self::SPECIFICATION_* to static::SPECIFICATION_* in non declarat...

This patch allow to extend \Zend\Db\Sql\Delete|Insert|Update classes in order to very easily implement driver specific syntaxes like the MySql REPLACE, INSERT IGNORE or INSERT ON DUPLICATE KEY UPDATE etc.

For example : 

```php
<?php
namespace Qapa\Db\Sql\Mysql;

class Replace extends \Zend\Db\Sql\Insert
{
    /**#@+
     * Constants
     *
     * @const
     */
    const SPECIFICATION_INSERT = 'replace';
    /**#@-*/

    /**
     * @var array Specification array
     */
    protected $specifications = array(
        self::SPECIFICATION_INSERT => 'REPLACE INTO %1$s (%2$s) VALUES (%3$s)'
    );
}
```

 or 

```php
<?php
namespace Qapa\Db\Sql\Mysql;

class InsertIgnore extends \Zend\Db\Sql\Insert
{
    /**#@+
     * Constants
     *
     * @const
     */
    const SPECIFICATION_INSERT = 'insertIgnore';
    /**#@-*/

    /**
     * @var array Specification array
     */
    protected $specifications = array(
        self::SPECIFICATION_INSERT => 'INSERT IGNORE INTO %1$s (%2$s) VALUES (%3$s)'
    );
}
```

If agreed, I will add unit tests.