JCLOUDS-160: Support tags in EC2 images

Added support for tags in EC2 images (based on the work done by @brockn in [jclouds#55](https://github.com/jclouds/jclouds/pull/55)).

I've built jclouds and all labs projects and all tests are passing.

@everett-toews @adriancole This affects aws-ec2 and openstack-nova-ec2. Could you have a quick look and see if this might break something?
@brockn Could you try these changes in your environment to make sure it still fits your needs?