Make sure that describe_volumes returns tags information

Hi,

I want to do some testing on tags but I noticed that moto currently doesn't return the Tagset for Volumes.  So I created some tests yesterday that verify whether tags are correclty returned.  Next I created a commit that implements this behavior.

Added a 3th commit to also include snapshots.  Also changed the logic that inheritence from the TaggedEC2Resource is used.

I ran nosetests and now they all succeed.

Kr,

Peter
I am not sure if I fully understand you.  If I look at the code I see that `EC2Backend` indeed inherits from `EBSBackend` so if `ec2_backend` is used everywhere there is indeed no problem.

What I don't understand is that in my initial PR I used `ec2_backend` instead of `self.ec2_backend` and due to this I would miss regional information.  Why don't I have the same issue now that I use `ec2_backend` upon initialization of Volume and Snapshot?

I did the proposed changes (or at least how I interpreted them :-) ).  But feel free to discard them if I missed the ball.