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