Utils\Html: attribute public accessor methods added

Closes #37 

New methods added & `::__call` refactored.
Full back-compatibility (see tests), new tests added.
I prefer `addAttribute()` too (instead of `appendAttribute()`).

`addAttributeValue()` is maybe unnecessary long - when I am adding attribute, I suppose that I can also add values. So I think `addAttribute()` is enough.

```php
Html::el('span')->addAttribute('class', 'one')->addAttribute('class', 'two');
Html::el('span')->addAttributeValue('class', 'one')->addAttributeValue('class', 'two');
```

What is confusing on `addAttribute()`, `setAttribute()`, `removeAttribute()` and `getAttribute()`?