Issue 128: Check if classes are available on classpath before loading

Fix for #128 

The resource set is created depending on the availability of the interface and implementation classes on the classpath. The names of the classes have changed since Gradle 2.2, both the plugin needs to support both pre-Gradle 2.2 and Gradle 2.2 and later versions.
Also expresses the intention better. Didn't know the GradleVersion.version property yet. Can I use it for a blog post ;-) ?