keyboardUp = function (key, callback) {
        var key_pressed;

        delete keysDown[key.keyCode];

        if (typeof key_map[key.keyCode] !== 'undefined') {
            key_pressed = getKeyPressed(key.keyCode);
            // Call user's noteDown function.
            callback(key_pressed, getFrequencyOfNote(key_pressed));
            darkenDown(document.getElementById(key_pressed));
        }
    }
Handle a keyboard key being released.
@param {element} key The DOM element of the key that was released.
@param {callback} callback The user's noteDown function.