figcaptions match image's width

the width of "figcaption" matches the width of the "figure" tag, while
preserving flexibility, responsivenes and keeping captions at the bottom
of the images. 

Based on a previous pull request for reverie 3 and referenced as a "inspiration for a neat solution" (quote: CherryFlavourPez) here: http://stackoverflow.com/questions/10264463/can-a-figcaption-be-restricted-to-the-width-of-a-responsively-sized-image