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()->addAttributes(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#'
));
```

Obviously there is no consensus inside ZF2 codebase when it comes to that ```@return``` annotation in case of some fluent-interface method which returns self reference. Somewhere you will see *self*, while on some other place name of the class or a fully qualified class name, like in case of this class. I prefer that variant, so I would rather leave it as is. At least until some coding standard for this purpose is defined. :)