Remove discontinued downloads-maven-plugin.
Update jetty-webapp from 8.1.2 to 9.1.3.

Changed the Java version, too. It's now a Java 7 project like the
jetty it uses.
Update dependencies.
Rename class Jetty to HttpServer.

We are useing Jetty, but this is an internal detail that should not
be exposed.
Reformat code according to IntelliJ's code style.
Update GPG Plugin from 1.4 to 1.5.
Allow arbitrary ports for the HTTP server.

There are enviroments where you cannot use port 8080.
Add configuration file for Travis CI.