Fix bug mixing R.java on Kotlin sources.

Fix https://github.com/cbeust/kobalt-android/issues/6

src/main/kotlin/com/beust/kobalt/plugin/java/JavaCompiler.kt

@@ -44,7 +44,11 @@ class JavaCompiler @Inject constructor(val jvmCompiler: JvmCompiler) {
                     allArgs.addAll(info.compilerArgs)
 
                     val fileManager = compiler.getStandardFileManager(null, null, null)
-                    val fileObjects = fileManager.getJavaFileObjectsFromFiles(info.sourceFiles.map { File(it) })
+                    val fileObjects = fileManager.getJavaFileObjectsFromFiles(info.sourceFiles.map {
+                            File(it)
+                        }.filter {
+                            it.isFile
+                        })
                     val dc = DiagnosticCollector<JavaFileObject>()
                     val classes = arrayListOf<String>()
                     val writer = PrintWriter(System.out)

src/main/kotlin/com/beust/kobalt/plugin/kotlin/KotlinPlugin.kt

@@ -121,6 +121,9 @@ class KotlinPlugin @Inject constructor(val executors: KobaltExecutors, val depen
 
         override val sourceDirectory = "kotlin"
 
+        /** kotlinc can be passed directories */
+        override val canCompileDirectories = true
+
         override fun compile(project: Project, context: KobaltContext, info: CompilerActionInfo): TaskResult {
             val result =
                     if (info.sourceFiles.size > 0) {