New YAML image structure – closes #115

There are only two commits here, but this has a significant effect on how the site is built. New YAML scheme looks like this:

~~~
image:
  - url: image.jpg
    caption: 'caption text'
    alt: 'alt text'
  - url: anotherimage.jpg
    caption: 'caption text'
    alt: 'alt text'
~~~

This change works with both photosets and writing posts.

Addresses #115