Fix broken code-blocks

[manuth]

Codeblock have an incorrect padding.

For example this here:

> **This is a blockquote**  
> ...with some code in it
> ```csharp
> Console.WriteLine("Hello World");
> ```

Renders like this.

As you can see the code-block is sticking on the bottom of the box which looks pretty weird.
This can be fixed by wrapping the code with a <div>-tag.

Additionally I updated the markdown.css to the newest version.

[yzane]

@manuth Thanks for your PR!