Add new Solarized Dark theme

After #2, here's a dark theme based on [Solarized](http://ethanschoonover.com/solarized) :

[![preview](https://raw.github.com/NumEricR/poche-themes/solarized-dark/solarized-dark/screenshot.jpg)](https://raw.github.com/NumEricR/poche-themes/solarized-dark/solarized-dark/screenshot.jpg)