replace long with int or ssize_t

1. Use `ssize_t` when reading file content or initialize with file content.
2. Use `int` instead of `long` for parameter and member variable in other places, including in `Vector` and `Map`. The reason of using `int` for container is that engine API will use the result after invoking container API, then it will cause more warnings. For example, 

```cpp
void Node::removeChild(Node* child, bool cleanup /* = true */)
{
    // explicit nil handling
    if (_children.empty())
    {
        return;
    }

    auto index = _children.getIndex(child);
    if( index != CC_INVALID_INDEX )
        this->detachChild( child, index, cleanup );
}
```

If the result of `_children.getIndex(child)` is `ssize_t`, then ` this->detachChild( child, index, cleanup )` will cause warnings.