add ASG-style tag list helper class

The troposphere.autoscaling.Tags helper class can be used much as the troposphere.Tags class, for the "Tags" field of an auto-scaling group.
The advantage is that you can set a default "PropagateAtLaunch" value (set to True in this case), and for each tag you can specifiy a different propagate property, using the following syntax: asg.Tags=autoscaling.Tags(Name='MyTag', Version=('10.20',False)).
The "MyTag" tag will have the "PropagateAtLaunch" property set to true, while the "Version" tag will have it set to false.