Add signing plugin.

build.gradle

@@ -8,6 +8,7 @@ subprojects {
 
     apply plugin: 'maven-publish'
     apply plugin: 'java'
+    apply plugin: 'signing'
 
     sourceSets {
         main {
@@ -90,6 +91,10 @@ subprojects {
         }
     }
 
+    signing {
+        sign configurations.archives
+    }
+
     sourceCompatibility = 1.7
     targetCompatibility = 1.7