Added slide deck logo feature to enable displaying a fixed logo image on...

If you would like to display a logo image on every slide, you can do so by setting ```slidedeckLogo``` configuration value to the URL of the image.

![screenshot from 2013-12-09 22 40 37](https://f.cloud.github.com/assets/118787/1709187/a221f716-611a-11e3-920e-e5891f1b7c7e.png)