Significant rewrite

* Removes a bunch of files that are unused
* Updates significantly to match current Chef workflows
* Renamespaces some attributes to make it a bit easier to wrap your head around
* Other cleanup

See commit messages for more detail. I'll probably do a minor version bump, as I believe this is backwards-compatible, unless particular attribute overrides were set. But I don't think that's worth a full version bump. I'm in the midst of testing this with test-kitchen.
Ah, sorry. I misunderstood. The semantics here are a bit confusing, but the original intent I think was that `jar_file` is the "automatically installed" jar, i.e. what you get from `gradle jar`. The `node[:exhibitor][:jar_dest]` value is where we want it to end up. In this case, the default recipe takes care of that for us. I think there are probably better ways to do this (at the very least, I can rename `jar_file` to `gradle_produced_jar` or something) so that one could call just this recipe and not default, but I'm not sure I want to mess with it too much in this PR.