Add option to Redcarpet::Render::HTML to emit style classes for google-code-prettify

I've added an option to the HTML renderer to add "prettyprint" classes to `<code>` blocks so that they can be formatted with google-code-prettify (http://code.google.com/p/google-code-prettify/).

I've changed the documentation, and added a unit test.

With a renderer instantiated like this:

```ruby
rnder = Redcarpet::Render::HTML.new(:prettify => true)
mkdwn = Redcarpet::Markdown.new(rnder, :no_intra_emphasis => true, :fenced_code_blocks => true, :space_after_headers => true)
mkdwn.render(text)
```
Code blocks will end up looking like this:
```html
<pre><code class="prettyprint">
…
</code></pre>
```