Plugin should be releaseable by any committer to repository

At the moment only Gradle core team can release new version as the release process requires credentials or secret tokens (e.g. to publish to GitHub pages). The plugin should define a release process that releases a new version whenever the code base has been tagged. Credentials will be defined as secret environment variables in Travis CI and used by the build process.

The release process should include the following functionality/steps:

- Version number is derived off of Git tag
- Create and publish the binary artifacts to the plugin portal
- Create and publish the documentation (user guide, Javadocs) to GitHub Pages
- Plugin version in samples are updated after the latest version has been release successfully
- Integrate process into Travis CI

As a result, all committers can release new versions without the need to wait on others with access to the credentials.