Added swap(index, index1) method

[davidefa]

Added swap method as follow:

	/*
		Swap objects at index and index1;
	*/
	virtual bool swap(int index, int index1);

template<typename T>
bool LinkedList<T>::swap(int index, int index1){
	// Check if index position is in bounds
	if(index < 0 || index >= _size)
		return false;
	if(index1 < 0 || index1 >= _size)
		return false;

	T _t = getNode(index)->data;
	getNode(index)->data = getNode(index1)->data;
	getNode(index1)->data = _t;
	return true;
}