Always use setters to fix memory issues without ARC

https://github.com/apache/cordova-plugin-file/pull/104

Encountered memory issues with a non-ARC project (cordova+unity3d). Setter should be used to prevent memory issues, eg:

A synthesized retained setter looks like :

- (void)setValue: (id)newValue
{
    if (value != newValue)
    {
        [value release];
        value = newValue;
        [value retain];
    }
}