Add hello-world sample

This pull request adds a simple hello-world sample to the samples directory. The sample *does not* utilize the existing template, instead opting to show the bare minimum necessary to add a Pedestal service to a Clojure project.

This commit also cleans up [samples/README.md](#diff-03a3756d2f8eaaf446a958595f62f98f), removing some non-existant samples and re-arranging samples in order skill level (roughly).