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_googletest_unittest/GildedRoseGoogletestUnitTests.cc

@@ -2,7 +2,7 @@
 #include "GildedRose.h"
 
 TEST(GildedRoseTest, Foo) {
-    vector<Item> items;
+    std::vector<Item> items;
     items.push_back(Item("Foo", 0, 0));
     GildedRose app(items);
     app.updateQuality();