Fix C++ code smells

* Move "using namespace std;" from header to module file (to no longer pollute the global namespace).
* Fix inconsistency of having namespace prefix in test code.
* Add include guard to GildedRose.h.
* Avoid std::endl.
* Have consistent white-space.

cpp/test/cpp_catch2_unittest/GildedRoseCatch2UnitTests.cc

@@ -3,7 +3,7 @@
 
 TEST_CASE("GildedRoseUnitTest", "Foo")
 {
-    vector<Item> items;
+    std::vector<Item> items;
     items.push_back(Item("Foo", 0, 0));
     GildedRose app(items);
     app.updateQuality();