Applied W3C valid standards syntax to +Twitter Cards. Valid by W3C and Twitter Cards Preview.
<meta name=twitter:card value=summary> syntax changed to
<meta property=twitter:card content=summary>
* The issue was cased on non-standard name attribute values.
* The Twitter Cards updated syntax had already corrected attribute value to content.
twitter:url changed to og:url, since og:url is wide recognized Open Graph protocol standard.
Added twitter:site to improve the snippet visual appearance.
Please, could you include this fixed on the next release?