Utils\Html: attribute public accessor methods added

Closes #37 

New methods added & `::__call` refactored.
Full back-compatibility (see tests), new tests added.
Hmm, thanks for explanation, I see it now :cry: 

My tips:
* `addAttributeValue()` is most descriptive, but in my opinion too long.
* `addValue()` is short, but it tells nothing about attributes.
* `appendAttribute()` sounds maybe good, but what about other new methods: `setAttribute()`, `removeAttribute()` and `getAttribute()`?
* `attr()` like jQuery (or `attribute()`), but it tells nothing about replacing/appending values...