Tests are added that verify that when a tag is being set on an (EBS) volume or on an instance that upon retrieval
from the resource, the tag are set on the instance.  Important is that the tags are set using create_tags but that
the presence is validated by getting the resource using either (get_all_volumes or get_all_instances).

When running the tests using nosetests it shows that the tags for the instances are correctly retrieved but the tags
for the volume are missing
Extend the DESCRIBE_VOLUMES_RESPONSE to include the tagSet as documented by AWS on http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-ItemType-DescribeVolumesSetItemResponseType.html .  This is needed to pass the test that was added in previous commit.
Extend the DESCRIBE_SNAPSHOTS_RESPONSE to include the tagSet as documented by AWS.  Also refactor the names of the tests I introduced in previous commits to be more descriptive.  Finally some code cleanup, removing the get_tags method from classes and instead inherit from the TaggedEC2Resource class.
Alter get_tags to use the backend associated with the taggable object.  Also give volume and snapshot an additional backend attribute.
Merge remote-tracking branch 'upstream/master'
Always use ec2_backend to get the tag information in order to have a cleaner get_tags method.
Merge pull request #258 from pvbouwel/master

Make sure that describe_volumes returns tags information