Call method on actual instance instead of class.

Use of `__CLASS__` disall extending of `Sioen\Converter` class. Original methods are executed instead of methods which override them. Methods not defined on `Sioen\Converter` were not found on extending class.

Since `toHtml` cannot be called staticaly, call method on instance.
Merge pull request #1 from Andreyco/patch-1

Call method on actual instance instead of class.