Fix some HTML5 syntax errors

This pull request should fix a certain amount of HTML5 syntax errors (yielded during a HTML validator check).

One bothering bug that remains is the addition of wrapping <p></p> tags around each {{ post.excerpt }} in index.html, i.e.

<p> {{ post.excerpt }}
</p>

resulting in

<p><p> lorem ipsum </p>
</p>

and therefore yielding a HTML5 syntax error.