Database: completly refactored SqlBuilder::addWhere()

- Brand new implementation `addWhere` method.
- Backward compatible!
- New options:

This code has already worked earlier:
```php
$table->where('nick', NULL); // `nick` IS NULL
$table->where('nick', "test"); // `nick` = "test"
$table->where('nick', array(1, 2, 3); // `nick` IN (1, 2, 3)
```
But not in this case:
```php
$table->where('nick ? OR nick ? OR nick ?', NULL, "test", array("a"));
// `nick` IS NULL OR `nick` = "test" OR `nick` IN ("a")
```

(Also full support for Selection and SqlLiteral)

Testers welcomed!