Add Image Support

[armahillo]

Enhancement for down the road...

Add the ability to incorporate small images into the cards. The image to use could be included as a line in the YAML ("/path/to/image.jpg") and the script could use one of the many image processing libs to ensure it's the correct dimensions. Incorporating into the HTML doc might require some tweaking -- perhaps we could export it as an MHTML archive or something?

Incorporating into the PDF would be easy enough -- there are image embedding functions in Prawn that I saw when working on the last PR last night.