Add meta tag which uses the property instead of name attribute

At the moment the MetaTag metadata field renders 

    <meta name="somename" content="somecontent" />

Unfortunately this doesn't play nice with Facebooks meta tag requirements which need something like:

    <meta property="somename" content="somevalue" />

So I've added a metadata field called `OgMetaTag` which renders as required but is otherwise identical to `MetaTag`.