Fixes #139 (Hanlon server config ignored by Rack-based servers)

This pull request includes changes to the Gemfile (to add a dependency on the Puma gem) and the `web/config.ru` file (to start up a Puma handler on the right port and under the right RESTful context). The changes in this PR have been tested when the server is started using both `trinidad` and `puma`, and they have also been tested when the Hanlon server is deployed as a WAR file under Tomcat.