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 
The entire contents of this Berksfile should just be able to be:

```ruby
source "https://api.berkshelf.com"

metadata
```

If `apt` is also a dependency, it should be added to `metadata.rb`.