Make the plugin applicable to Settings

This change makes the plugin applicable to a Settings instance
in settings.gradle. As far as I can tell, the issue from
http://forums.gradle.org/gradle/topics/applying_a_plugin_by_alias_to_settings_gradle
is a Gradle bug and not a problem of my implementation, so it would
be great if you could merge this change in also before releasing,
as it is already usable, you just have to use the FQCN to apply the
plugin in settings.gradle as I noted in the README.md.