Create ex3_21_generics_version

To replace ex3_21.cpp 
This one is generics version using template, for general version please check @pezy's code.

ch03/ex3_21_generics_version

@@ -0,0 +1,44 @@
+//!
+//! @author Yue Wang
+//! @date   4th Sep 2014
+//!
+
+#include <iostream>
+#include <vector>
+
+/**
+ * @brief check
+ * @param first
+ * @param last
+ *
+ * generics version for ex3.21
+ */
+template<typename Iter>
+void check(Iter first, Iter last)
+{
+    for(auto it = first; it != last; ++it)
+        std::cout << *it << "\n";
+
+    std::cout << "size = " << last - first << "\n\n";
+}
+
+int main()
+{
+    std::vector<int> v1;
+    std::vector<int> v2(10);
+    std::vector<int> v3(10, 42);
+    std::vector<int> v4{10};
+    std::vector<int> v5{10, 42};
+    std::vector<std::string> v6{10};
+    std::vector<std::string> v7{10, "hi"};
+
+    check(v1.begin(),v1.end());
+    check(v2.begin(),v2.end());
+    check(v3.begin(),v3.end());
+    check(v4.begin(),v4.end());
+    check(v5.begin(),v5.end());
+    check(v6.begin(),v6.end());
+    check(v7.begin(),v7.end());
+
+    return 0;
+}