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_approvaltest/GildedRoseGoogletestApprovalTests.cc

@@ -12,12 +12,12 @@ std::ostream& operator<<(std::ostream& os, const Item& obj)
 
 TEST(GildedRoseApprovalTests, VerifyCombinations) {
 
-    std::vector<string> names { "Foo" };
+    std::vector<std::string> names { "Foo" };
     std::vector<int> sellIns { 1 };
     std::vector<int> qualities { 1 };
 
-    auto f = [](string name, int sellIn, int quality) {
-        vector<Item> items = {Item(name, sellIn, quality)};
+    auto f = [](std::string name, int sellIn, int quality) {
+        std::vector<Item> items = {Item(name, sellIn, quality)};
         GildedRose app(items);
         app.updateQuality();
         return items[0];