Photo post types, modular enhancements and other significant changes - resolves #36 #47 #49

Time for some code review. Loads of changes here.

- New post category: photo
- Excerpts for photos (uses a neat liquid for loop trick)
- Includes for excerpts (more maintainable, can reuse these items now)
- Previous/next buttons work per-category
- Clearer titles for includes
- Clearer titles for layouts
- More flexible YAML (e.g. using collection arrays for images)
- Standardized YAML front matter
- Standardizing post images at 960px and photo images at 1024px
- Wider images in writing posts
- Starting to work on alt text for images via YAML
- Significantly refactored CSS
- Better HTML class names
- More maintainable and modular HTML