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.
modified code to be in conformity with pep8
Merge pull request #165 from lapin10/patch-1

add ASG-style tag list helper class