Add Gradle plugin for automatic Sonatype releases. (#788)

build.gradle

@@ -20,6 +20,9 @@ buildscript {
         // Kotlin
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
 
+        // Automatic releases to Sonatype.
+        classpath 'io.codearte.gradle.nexus:gradle-nexus-staging-plugin:0.8.0'
+
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
     }
@@ -149,3 +152,8 @@ subprojects {
     }
 }
 
+apply plugin: 'io.codearte.nexus-staging'
+
+nexusStaging {
+    packageGroup = 'com.pushtorefresh'
+}