Utils\Html: attribute public accessor methods added

Closes #37 

New methods added & `::__call` refactored.
Full back-compatibility (see tests), new tests added.
This allows merge new values with previous, see test: https://github.com/nette/utils/pull/38/files#diff-010f6571f7fdc6a1430059e0a2656850R51 (line 51 - 62)