[java-decompiler] migrates to Gradle

Simplifies CI (the outer IDEA project no longer bundles JUnit and AssertJ .jars).

build.gradle

@@ -0,0 +1,24 @@
+apply plugin: 'java'
+
+compileJava {
+  sourceCompatibility '1.8'
+  targetCompatibility '1.8'
+}
+
+sourceSets {
+  main.java.srcDirs 'src'
+  test.java.srcDirs 'test'
+}
+
+repositories { jcenter() }
+dependencies {
+  testCompile 'junit:junit:4.+'
+  testCompile 'org.assertj:assertj-core:3.+'
+}
+
+jar {
+  archiveName 'fernflower.jar'
+  manifest {
+    attributes 'Main-Class': 'org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler'
+  }
+}