[#34027] Add ability to override the renderer [New feature]

In Joomla! if you want to override something you have to put it in the active template directory (e.g.: the layouts goes in (active template)/html/layouts).
This PR introduces the ability to override the whole rendering process in the template by copying (and modifying per your preferences) the /libraries/joomla/document directory.

It shouldn't rise any backward compatibility issues (except that the directory [active template]/document might be in use?).

What this PR might provide to Joomla! devs/users?
Flexibility on the way that the template renders the data (the inner logic). e.g. someone may use ```echo $this->getBuffer('modules', 'menu', array('menu', 'html5'));``` and remove the ```<jdoc.. >``` parser.

Why?
Because this is one area that if you want to override and use your own logic you have to do it through a plugin (this is way easier and cleaner)

Feature tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=34027

I'm wondering if it could make sense to check for the actual renderer file instead of checking the folder?
This way you could only override one specific renderer and it would gracefully fall back to the default renderer if the specific type isn't available. At least I think it would :)