Refactor into a more "true" Application Cookbook

- [ ] the default recipe should be have as little as possible except for `include_recipe`
- [ ] rename recipes with the _ syntax, because they shouldn't be in the run-list of a node.
- [ ] remove any resources that are specific configuration ; they belong in a wrapper cookbook that uses `nexus::default`
- [ ] update README big time.
- [ ] rework logic and flow for either nginx or jetty.