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.