Change method of recognizing raw html blocks.

I was not able to add some raw html to the markdown. For example to embed Flickr's slideshows to the page one has to insert <object> tag that contains symbol '@` somewhere within. Current implementation treats such html as text paragraph.

I've changed the algorithm of recognizing html blocks. It now corresponds to the syntax of block-level HTML elements in the [standard markdown](http://daringfireball.net/projects/markdown/syntax#html).