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
Okay, I believe the current code is correct, but we can clean it up a bit.

`EBSBackend` is really more of a mixin than a standalone backend. It is subclassed by `EC2Backend`. This means that the actual  `ec2_backend` instance we use already supports the `EBSBackend` methods.

So, for the Volume and Snapshot instantiation, we should be able to just say `self.ec2_backend = ec2_backend` and then be able to cleanup the `get_tags` method. Does this make sense? I know the setup here is a bit confusing. I can also merge it and clean this last bit up by myself.