Add method TaggedEC2Object.add_tags().

There is already a `TaggedEC2Object.add_tag()` method but it only supports
adding one tag at a time. The [CreateTags](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CreateTags.html) api supports adding multiple tags
at once. So, when you want to add multiple tags you needlessly make multiple
round-trips to EC2 service. The `add_tags()` method helps in avoiding that.