Allow overriding virtual functions per instance.

This changes the way virtual function overrides are fetched so that it
is possible to override them directly on an instance, rather than
creating a derived class in Lua:

  x = X()
  x.virtual_function = function(self) ... end

src/wrapper_base.cpp

@@ -34,12 +34,10 @@ namespace luabind { namespace detail
 	LUABIND_API void do_call_member_selection(lua_State* L, char const* name)
 	{
 		object_rep* obj = static_cast<object_rep*>(lua_touserdata(L, -1));
-		lua_pop(L, 1); // pop self
 
-		obj->crep()->get_table(L); // push the crep table
-		lua_pushstring(L, name);
-		lua_gettable(L, -2);
-		lua_remove(L, -2); // remove the crep table
+        lua_pushstring(L, name);
+        lua_gettable(L, -2);
+        lua_replace(L, -2);
 
 		if (!is_luabind_function(L, -1))
 			return;

test/test_abstract_base.cpp

@@ -110,5 +110,13 @@ void test_main(lua_State* L)
         "y:hello()\n");
 
     DOSTRING(L, "call_hello(y)\n");
+
+    DOSTRING(L,
+        "x = abstract()\n"
+        "x.hello = function(self) return 'hello from instance' end\n"
+        "print(x.hello)\n"
+        "assert(x:hello() == 'hello from instance')\n"
+        "assert(call_hello(x) == 'hello from instance')\n"
+    );
 }