Gradle tweaks: fix for Oracle javadoc doclint strictness; made the signing task a bit more intelligent.

api/build.gradle

@@ -25,6 +25,8 @@ artifacts {
 }
 
 signing {
+    // Only sign if we're uploading...
+    required { gradle.taskGraph.hasTask("uploadArchives") }
     sign configurations.archives
 }
 

build.gradle

@@ -0,0 +1,7 @@
+if (JavaVersion.current().isJava8Compatible()) {
+  allprojects {
+    tasks.withType(Javadoc) {
+      options.addStringOption('Xdoclint:none', '-quiet')
+    }
+  }
+}