Migrated core and extension modules to Kotlin

(Except BitmessageContext and Bytes)

build.gradle

@@ -1,9 +1,19 @@
+buildscript {
+    ext.kotlin_version = '1.1.2-2'
+    repositories {
+        mavenCentral()
+    }
+    dependencies {
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+    }
+}
 plugins {
     id 'com.github.ben-manes.versions' version '0.14.0'
 }
 
 subprojects {
     apply plugin: 'java'
+    apply plugin: 'kotlin'
     apply plugin: 'maven'
     apply plugin: 'signing'
     apply plugin: 'jacoco'
@@ -17,6 +27,9 @@ subprojects {
         mavenCentral()
         maven { url 'https://oss.sonatype.org/content/repositories/snapshots/' }
     }
+    dependencies {
+        compile "org.jetbrains.kotlin:kotlin-stdlib-jre8:$kotlin_version"
+    }
 
     test {
         testLogging {