Compatibility with serialized_attributes gem

Hi,

I've been trying to use Paper Trail with the [serialized_attributes gem](https://github.com/technoweenie/serialized_attributes). After much pain and anguish, I figured out that both Paper Trail and SerializedAttributes define the same class method on the model: ```serialize_attributes```. You can image the havoc this caused. =)

I think the best solution is to change the method names in Paper Trail. This is because the ```serialized_attributes``` method in the Serialized Attributes gem is a public interface. You must call this method in order to enable the gem. In Paper Trail, this method is only used internally. So it should be safe to change the method names in Paper Trail. Another advantage is that ```serialize_attributes``` is a pretty generic method name, which may interfere with other gems, so it's good to make the name more specific to Paper Trail.