Factor out Exhibitor

This PR is a massive change to accommodate the new [Exhibitor cookbook](). It will be a full version bump to 2.0.0, but is now a pre-release. I'll test for the next day or two both this and Exhibitor (and clean up both), and I intend to make this a LWRP, probably maintaining a default recipe to keep backwards-compatibility.

Supercedes #76 
This is a bit out-of-scope for this changeset, but it appears to have landed in here, so what the hell.

This looks like a Berkshelf 2 Berksfile. I can't encourage highly enough using Berkshelf 3; primarily b/c Berkshelf 3 offers much better dependency resolution ([v3.0.0 changelog here](https://github.com/berkshelf/berkshelf/blob/master/CHANGELOG.md#300), but also because Berkshelf 2 is deprecated.

You can check out the [Berkshelf 2 → 3 upgrade guide](https://github.com/berkshelf/berkshelf/wiki/2.x-to-3.0-Upgrade-Guide) for how to tweak this.

Also, no need to specify cookbooks like the below; you can just use the `metadata` method, and it will read your `metadata.rb` file, thus allowing you to DRY things up.