Create a tagging mechanism

[thaljef]

This idea started life as #142.

Basically, create a mechanism for giving symbolic names to revisions, similar to git tag. The following operations should then be possible:

• pinto tag 'my-tag-name' # tag current stack by name 'my-tag-name'
• pinto revert 'my-tag-name' # revert to tag named 'my-tag-name', similar to pinto revert revision ?
• pinto copy stack1 --tag 'my-tag-name' stack2 # the same as pinto copy command, but copy tag not head of stack1 to stack2 - in my workflow would be usefull if I want to copy a build from one project to another project
• pinto diff 'tag1' 'tag2' # the same as pinto diff but between tags in one stack

Issues to consider:

• Distinguishing tag names from stack names and revision ids.
• Tag attributes (name, creator, description, mtime).
• Allowing multiple tags per revision.
• Deleting/renaming tags.