color schemes

This pull req attempts to address #7 (Optional inverted color scheme). Currently there's no UI to select the color theme, instead `pamflet.js` looks for query parameter `color_scheme` on load.

The following is from the newly added docs with slight modification (links point to tetrix pamflet instead of itself).

Color Schemes
-------------

You might want to read pamflets using a dark color scheme. That's why Pamflet ships with three color schemes: <a href="http://eed3si9n.com/tetrix-in-scala/concurrency.html?color_scheme=github">github</a>, <a href="http://eed3si9n.com/tetrix-in-scala/concurrency.html?color_scheme=monokai">monokai</a>, and <a href="http://eed3si9n.com/tetrix-in-scala/concurrency.html?color_scheme=redmond">redmond</a>.

### Applying a color scheme

To apply a dark color scheme, open your pamflet with query string
<nobr><code>?color_scheme=monokai</code></nobr>. This will store `"monokai"` to the HTML5 local storage, and apply the scheme to the pamflet.

### Default color scheme

The default scheme is github. To change it to something else, set `color_scheme` property:

    color_scheme=monokai

### Custom color scheme

To create your own custom color scheme, for example named "zen", create `color_scheme-zen.css`
and declare css properties under `body.color_scheme-zen`:

```css
body.color_scheme-zen {
    color: black;
    background: white;
}
body.color_scheme-zen code.prettyprint span.str {
  color: #dd1144
}
```