use class instead of a module and add basic test

ext/hello/hello.c

@@ -1,15 +1,14 @@
 #include "ruby.h"
 
+VALUE hello_world(VALUE self)
+{
+    return rb_str_new2("Hello, World!");
+}
+
 VALUE Hello;
-VALUE hello_world(VALUE self);
 
 void Init_hello()
 {
-    Hello = rb_define_module("Hello");
+    Hello = rb_define_class("Hello", rb_cObject);
     rb_define_method(Hello, "world", hello_world, 0);
-}
-
-VALUE hello_world(VALUE self)
-{
-    return rb_str_new2("Hello, World!");
 }

lib/hello.rb

@@ -1,5 +1,5 @@
 require 'hello/hello'
 
-module Hello
+class Hello
   VERSION = '0.0.1'
 end

test/hello_test.rb

@@ -0,0 +1,8 @@
+require 'minitest/autorun'
+require 'hello/hello'
+
+class HelloTest < MiniTest::Unit::TestCase
+  def test_sanity
+    assert_equal 'Hello, World!', Hello.new.world
+  end
+end