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 agree but if I change only the "self."  the code won't work for volumes and snapshots since those don't have a ec2_backend property.  I have added an ebs_backend to the Volume and Snapshot objects and have altered the get_tags code a little bit more but I am not sure if this is a valid way to go, feedback is welcome.