Release 1.6.0

[dmitrykuzmin]

This PR does the release of a bootstrap plugin version 1.6.0.

With this release the bootstrap plugin receives support for more automated project configurations [#65].

Library dependencies

It's now possible to add some of the Spine libraries to project dependencies through the API provided by the plugin. This includes:

• the Google Cloud Datastore storage implementation:

spine.enableJava {
    withDatastore()
}

• web API library:

spine.enableJava().webServer()

• web library with integration over the Firebase Realtime Database:

spine.enableJava().firebaseWebServer()

IDEA configuration

If both Bootstrap and the idea plugin are applied, Bootstrap configures the idea plugin, so that the IDE displays sources and generated sources as expected.

Infrastructure

The project build scripts are migrated to Kotlin [#50].

[armiol]

@dmitrykuzmin LGTM. However, let's inline (and maybe shorten a bit) the description from #65 into the notes. It looks very helpful to me if we'd have the sample usage along with the explanation right in the release notes.