Add a gradle task for building the project into a jar file

[damithc]

Some students use AB-3 as the basis for their project. They need a gradle task to build their product as a jar file.

Fix suggested by @pyokagan

diff --git a/build.gradle b/build.gradle
index 0a42902..255ef06 100644
--- a/build.gradle
+++ b/build.gradle
@@ -7,8 +7,13 @@ import org.gradle.api.tasks.testing.logging.TestLogEvent
 plugins {
     id 'java'
     id 'org.asciidoctor.convert' version '1.5.6'
+    id 'com.github.johnrengelman.shadow' version '2.0.3'
+    id 'application'
 }

+// Specifies the entry point of the application
+mainClassName = 'seedu.addressbook.Main'
+
 sourceCompatibility = JavaVersion.VERSION_1_9
 targetCompatibility = JavaVersion.VERSION_1_9

@@ -41,6 +46,12 @@ dependencies {
     testImplementation group: 'junit', name: 'junit', version: '4.12'
 }

+shadowJar {
+    archiveName = 'addressbook.jar'
+
+    destinationDir = file("${buildDir}/jar/")
+}
+
 task wrapper(type: Wrapper) {
     gradleVersion = '4.8.1'
 }