Some basic styling for figures and their captions.

There was no styling for the figure and figcaption elements yet, and the default spacing seemed a little too tight for floating figures. You can see this (in the floatright version) in action on my blog:
http://lappland.io/blog/2014/01/data-flirting-or-saving-beautiful-tree/

The corresponding code [can be found here](https://github.com/hlapp/hlapp.github.io/blame/622c3d64c843d91773406e87f03481c60071f3c0/content/articles/data-flirting-or-beautiful-tree.md#L22-25).