RFC 17 implementation

This branch implements [Chef RFC 17](https://github.com/opscode/chef-rfc/blob/master/rfc017-file-specificity.md). I've only run the unit tests as many of the others seem to take a very long time and some break due to TTY issues:

```
Finished in 2 minutes 14.7 seconds
6864 examples, 0 failures, 30 pending
```
Also not super sure which mixin you mean. This could be done in the resource classes I think, but that would require some funky code to transform the input which would probably be confusing or it could be done in the Content classes but those don't share any mixin right now.