Does anyone know how to get a character to rotate and move in the direction of the key they press?

Like pressing the left key makes the character look left and move that way?

using UnityEngine;    
using System.Collections;

public class Move : MonoBehaviour{          
    public float speed = 6.0F;    
    public float jumpSpeed = 8.0F;    
    public float gravity = 20.0F;        

    private Vector3 moveDirection = Vector3.zero;       

    void Update() {
        CharacterController controller = GetComponent<CharacterController>();
        if (controller.isGrounded) {
            moveDirection = new Vector3(0, 0, Input.GetAxis("Horizontal"));    
            moveDirection = transform.TransformDirection(moveDirection);    
            moveDirection *= speed;    
            if (Input.GetButton("Jump"))    
                moveDirection.y = jumpSpeed;    
        }

        moveDirection.y -= gravity * Time.deltaTime;    
        controller.Move(moveDirection * Time.deltaTime);
    }    
}

I tried this

 else if (Input.GetKeyDown(KeyCode.A))
        transform.forward = new Vector3(1f, 0f, 0f);
    else if (Input.GetKeyDown(KeyCode.D))
        transform.forward = new Vector3(-1f, 0f, 0f);

The turning direction is fixed but it still moves in only one direction.

[Edit Berenger : Code formatting]