Added tag support to Volumes and Snapshots. 

Adds:
* 'update tags'
* 'remove tags'
* 'filter by tags'
... to EBSVolume and EBSSnapshot.

Also:
* Re-factored tag functionality in AMI/EC2 and moved some common code to AWSCloud.
* Added check to AWSCloud before trying to add tags to resource if no tags were specified.