Add support for Kotlin in unit tests

Author: marcinqualaroo

build.gradle

@@ -1,9 +1,19 @@
+buildscript {
+    ext.kotlin_version = '1.2.61'
+    repositories {
+        mavenCentral()
+    }
+    dependencies {
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+    }
+}
 plugins {
     id 'com.github.kt3k.coveralls' version '2.6.3'
     id 'com.jfrog.bintray' version '1.7.3'
 }
 
 apply plugin: 'checkstyle'
+apply plugin: 'kotlin'
 apply plugin: 'jacoco'
 apply plugin: 'java-library'
 apply plugin: 'maven'
@@ -25,6 +35,7 @@ dependencies {
     testImplementation 'junit:junit:4.12' // Testing
     testImplementation 'org.mockito:mockito-core:2.8.47' // Mocking
     testImplementation 'com.squareup.retrofit2:retrofit-mock:2.3.0' // Helpers for Retrofit
+    testImplementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"
 
     compile 'com.squareup.okhttp3:okhttp:3.8.0' // Low-level HTTP client
     compile 'com.squareup.retrofit2:retrofit:2.3.0' // High-level HTTP client
@@ -35,7 +46,8 @@ dependencies {
         exclude group: 'com.google.code.findbugs', module: 'jsr305'
     })
 
-    compile 'io.reactivex.rxjava2:rxjava:2.1.2' // Observable pattern for async methods
+    compile 'io.reactivex.rxjava2:rxjava:2.1.2'
+    compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version" // Observable pattern for async methods
 }
 
 javadoc {
@@ -150,3 +162,9 @@ jacocoTestReport {
         html.enabled = true
     }
 }
+
+compileTestKotlin {
+    kotlinOptions {
+        jvmTarget = "1.8"
+    }
+}