added support for dynamic resizing

This changeset adds the ability to support full height/width images in the browser window.
It does so by checking the current image's size against the browser viewport size, and then match the longest axis with the browser's.
To select dynamic fullscreen size of images, just set up `height = "dynamic"` in the setup of the portfolio gallery.

This code should fix #19

Signed-off-by: Guyzmo <guyzmo+github@m0g.net>