[Go] Revert commit 5e88857 to undelete the 'callback' and 'extend' examples.

The 'callback' and 'extend' examples were presumed to be obsoleted by the new
'director' example.  The examples are helpful though to have similar examples
across target languages and hence the commit @5e88857 which removed these
examples got reverted.

Examples/go/extend/example.h

@@ -0,0 +1,56 @@
+/* File : example.h */
+
+#include <cstdio>
+#include <iostream>
+#include <vector>
+#include <string>
+#include <cmath>
+
+class Employee {
+private:
+	std::string name;
+public:
+	Employee(const char* n): name(n) {}
+	virtual std::string getTitle() { return getPosition() + " " + getName(); }
+	virtual std::string getName() { return name; }
+	virtual std::string getPosition() const { return "Employee"; }
+	virtual ~Employee() { printf("~Employee() @ %p\n", (void *)this); }
+};
+
+
+class Manager: public Employee {
+public:
+	Manager(const char* n): Employee(n) {}
+	virtual std::string getPosition() const { return "Manager"; }
+};
+
+
+class EmployeeList {
+	std::vector<Employee*> list;
+public:
+	EmployeeList() {
+		list.push_back(new Employee("Bob"));
+		list.push_back(new Employee("Jane"));
+		list.push_back(new Manager("Ted"));
+	}
+	void addEmployee(Employee *p) {
+		list.push_back(p);
+		std::cout << "New employee added.   Current employees are:" << std::endl;
+		std::vector<Employee*>::iterator i;
+		for (i=list.begin(); i!=list.end(); i++) {
+			std::cout << "  " << (*i)->getTitle() << std::endl;
+		}
+	}
+	const Employee *get_item(int i) {
+		return list[i];
+	}
+	~EmployeeList() { 
+		std::vector<Employee*>::iterator i;
+		std::cout << "~EmployeeList, deleting " << list.size() << " employees." << std::endl;
+		for (i=list.begin(); i!=list.end(); i++) {
+			delete *i;
+		}
+		std::cout << "~EmployeeList empty." << std::endl;
+	}
+};
+