Database: Named parameters

Sometimes I need to use query parameters elsewhere than in "where". It was quite difficult, but this https://github.com/nette/nette/pull/945 makes it impossible.

Now it is not only possible, but much easier.

Example:
```php
$selection->where("MATCH(content) AGAINST(:search)");
$selection->order("MATCH(content) AGAINST(:search) DESC");
$selection->parameter('search', $search);
```