Add signing of distributions (#520)

Executing `./gradlew metafacture-runner:signArchives` builds signed
distributions in metafacture-core/metafacture-runner/build/distributions/.

Author: dr0i

metafacture-runner/build.gradle

@@ -19,6 +19,7 @@ ext.mavenName = 'Metafacture Runner'
 description = 'CLI application for data processing with Flux and Metamorph'
 
 apply plugin: 'distribution'
+apply plugin: 'signing'
 
 configurations {
   plugins {
@@ -144,6 +145,10 @@ distTar {
   archiveExtension = 'tar.gz'
 }
 
+signing {
+  sign configurations.archives
+}
+
 distZip {
   archiveClassifier = 'dist'
 }