Use sbt-release plugin for release management

Use [sbt-release](https://github.com/sbt/sbt-release) plugin to automate future releases. I couldn't test it out completely as I've got no access to publish plugins, esp. sbt-web.

To fire it up, use `sbt release`. Not sure if there's a test run, but it could be tested for the upcoming release(s). It worked for me for another sbt project, but without publishing to remote repositories. Worth to give it a shot anyway.