New theme created - Gum

Hi Pelican team!

I've created a new theme for Pelican called 'Gum', based on the [Gumby CSS Framework](http://gumbyframework.com/docs)

The theme is minimal in terms of design and also responsive.

Links / Screenshots:
 - [Demo site](http://nickrance.co.uk/pelican/)
 - [Screenshot 1 - Main Design](https://github.com/uknick/pelican-themes/blob/pelican-flatui/gum/screenshot.png)
 - [Screenshot 2 - Typography](https://github.com/uknick/pelican-themes/blob/pelican-flatui/gum/typography.png)

Nick