java_gen: create OFFactory

first shot at factory interface for OF Objects. Will have to be
refined later.
diff --git a/java_gen/templates/_singleton.java b/java_gen/templates/_singleton.java
new file mode 100644
index 0000000..8ea9f01
--- /dev/null
+++ b/java_gen/templates/_singleton.java
@@ -0,0 +1,10 @@
+
+    private ${msg.name}() {}
+
+    private final static class Holder {
+        private static final ${msg.name} INSTANCE = new ${msg.name}();
+    }
+
+    public static ${msg.name} getInstance() {
+        return Holder.INSTANCE;
+    }