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
```
So yeah, if we want to do it that way, I would vote for a mixin included into `Chef::Provider::CookbookFile::Content` and `Chef::Provider::Template::Content`. I'm not sure thats really any better though, just a different way to organize things. Currently all the file lookup logic is in `CookbookVersion` and it seems cleaner to leave it that way and make the Content classes just blindly request paths.