#84 - Made constructor in Resource package private.

This change is to essentially prevent people from subclassing Resource and call the default constructor which wrecks the assumption of content never being null. For custom resources extend ResourceSupport.

Added missing license header in ResourcesMixin.

src/main/java/org/springframework/hateoas/Resource.java

@@ -36,7 +36,7 @@ public class Resource<T> extends ResourceSupport {
 	/**
 	 * Creates an empty {@link Resource}.
 	 */
-	protected Resource() {
+	Resource() {
 		this.content = null;
 	}