use jekyll-image-tag to generate resized images

Make use of Rob Wierzbowski's jekyll-image-tag plugin [0] to generate
thumnails for album pages and medium sized images for the image pages
(also link image page images to the full-sized images)

I'll need to figure out how to make a yaml file that I can just include
in a site's _config.yml or something. In the mean time, my
jekyll-image-tag config is reproduced below:

---
image:
    output: generated
    presets:
        album_thumb:
            width: 200
            attr:
                class: album-image
        medium:
            width: 1200
            attr:
                class: image
---

[0] https://github.com/robwierzbowski/jekyll-image-tag

Fixes #24