HtmlTag helper

New view helper used for rendering `<html>` tag of a web page, to which some custom attributes can be added dynamically.

## Intent
Idea is to have ability to dynamically attach some attributes to the opening HTML tag in a layout. For example:
```<html xmlns="http://www.w3.org/1999/xhtml" xmlns:og="http://ogp.me/ns#">```

## Usage
Rendering tag in a layout:
```php
echo $this->html()->openTag(); 
//...
echo $this->html()->closeTag();
```

Adding attributes:
```php
//single
$this->html()->setAttribute('xmlns', 'http://www.w3.org/1999/xhtml');

//multiple
$this->html()->setAttributes(array(
    'xmlns' => 'http://www.w3.org/1999/xhtml',
    'prefix' => 'og: http://ogp.me/ns#'
));

//shortcut method
$this->html(array(
    'xmlns' => 'http://www.w3.org/1999/xhtml',
    'prefix' => 'og: http://ogp.me/ns#'
));
```

There is also option to automatically pre-set appropriate namespace attributes, based on currently set DOCTYPE (`Doctype` helper):
```php
//At some point:
$this->doctype('xhtml');

$this->html()
    ->setUseNamespaces(true) //'xmlns' attribute will be automatically set to 'http://www.w3.org/1999/xhtml'
    ->setAttributes(array(
        'prefix' => 'og: http://ogp.me/ns#'
    ));
```
No, it will be done after merge. I don't feel like burdening existing PRs with these changes is needed at all ;-)