HeadMeta plugins

## Problem
Common use-case of the HeadMeta view helper is adding some meta tags, like those that are defined by the [Open Graph](http://ogp.me) specification. As a result, some repetitive syntax might be spread throughout the code, for example:
```php
$headMeta->setProperty('og:type', 'website');
$headMeta->setProperty('og:title', 'Some title');
$headMeta->setProperty('og:description', 'Some description');
```

## Solution
Feature which extends API of the HeadMeta view helper in such manner that custom **plugins** can be hooked to the default __call() routine, in order to have **custom "virtual" methods**.

### Concrete implementation
This proposal also introduces concrete plugin implementation which facilitates usage of the above mentioned **OpenGraph** meta tags. For example:
```php
$headMeta->registerPlugin('opengraph');
$headMeta->setOgTitle('My page title'); //<meta property="og:title" content="My page title">
$headMeta->setOgTitle('some description'); //<meta property="og:description" content="some description">
$headMeta->setOgType('article'); //<meta property="og:type" content="article">
//Custom type notation (underscore):
$headMeta->setOg_ArticleAuthor('http://www.example.com/author/1'); //<meta property="article:author" content="http://www.example.com/author/1">
```