PlayerMovement.cs

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    // Debugging
    public bool loggingEnabled = false;

    // Keycodes
    public KeyCode moveLeft = KeyCode.A;
    public KeyCode moveRight = KeyCode.D;
    public KeyCode jump = KeyCode.Space;

    public KeyCode altMoveLeft = KeyCode.LeftArrow;
    public KeyCode altMoveRight = KeyCode.RightArrow;
    public KeyCode altJump = KeyCode.Z;

    // Physics
    public float acceleration = 10;
    public float jumpForce = 200;
    public float maxSpeed = 10;

    private Vector2 moveDir = Vector2.zero;
    // By doing this, isGrounded becomes read-only outside of this script
    [HideInInspector] public bool isGrounded { get; private set; }

    // Initialization
    void Start ()
    {
        isGrounded = false;
    }

    void FixedUpdate ()
    {
        moveDir = Vector2.zero;

        if (isGrounded)
        {
            if (Input.GetKey (jump) || Input.GetKey (altJump) && rigidbody2D.velocity.y < 10)
                moveDir.y += jumpForce;
        }

        if (Input.GetKey (moveLeft) || Input.GetKey (altMoveLeft))
        {
            moveDir.x -= acceleration;
        }

        if (Input.GetKey (moveRight) || Input.GetKey (altMoveRight))
        {
            moveDir.x += acceleration;
        }

        rigidbody2D.AddForce (moveDir);

        Vector2 velocity = rigidbody2D.velocity;
        velocity.x = Mathf.Clamp (velocity.x, -maxSpeed, maxSpeed);
        rigidbody2D.velocity = velocity;

        if (loggingEnabled)
            Debug.Log (isGrounded + " " + transform.position + " | " + moveDir);
    }

    // Thanks to Thenewbloods for the info
    void OnTriggerStay2D (Collider2D other)
    {
        if (other.tag == "Ground" && isGrounded == false)
            isGrounded = true;
    }

    void OnTriggerExit2D (Collider2D other)
    {
        if (other.tag == "Ground")
            isGrounded = false;
    }
}