Compatibility with jekyll 1.0 and refactoring

plugin not working in jekyll 1.0 branch:

```
nazz.me #dev > jekyll serve
Configuration file: /Library/WebServer/Documents/nazz.me/_config.yml
error: wrong argument type Class (expected Module). Use --trace to view backtrace
nazz.me #dev > jekyll serve --trace
Configuration file: /Library/WebServer/Documents/nazz.me/_config.yml
/Library/WebServer/Documents/nazz.me/_plugins/related_posts.rb:63:in `include': wrong argument type Class (expected Module) (TypeError)
```

Plus slightly simplified plugin structure.