Fix to remove the closing link tag that was causing pages to fail HTML5 W3C validation.

Links generated by the asset pipeline (in specific by CssAsset) currently look like this:

`<link rel="stylesheet" type="text/css" href="/path/to/where/ever/styles.css"></link>`

To conform with [the HTML5 spec](http://www.w3.org/html/wg/drafts/html/master/document-metadata.html#attr-link-type) they should omit the trailing `</link>` which is currently causing documents to fail validation. 

I've implemented a new `HeadLink` HTMLObject to achieve this as the protected method `$isSelfClosing` needed setting. I've also created as static creator and fixed up a constructor with the ability to also easily set the 'media' attribute, hopefully this could be implemented when registering assets down the line as it would be helpful to be able to control this when attaching CSS assets.

The links now look like this:
`<link href="/path/to/where/ever/styles.css" rel="stylesheet" type="text/css" media="all">`