Add jar manifest to gradle build

Author: sloenthran

build.gradle

@@ -14,3 +14,11 @@ repositories {
 dependencies {
     testCompile group: 'junit', name: 'junit', version: '4.12'
 }
+
+jar {
+    manifest {
+        attributes(
+                'Main-Class': 'pl.nogacz.checkers.Checkers',
+        )
+    }
+}