Merge pull request #24 from clstokes/develop

Fixed NullPointerException when adding a tag with a null value. Added log error statement to catch in the future.
Merge branch 'develop' of https://github.com/greese/dasein-cloud-aws into develop
Merge pull request #25 from clstokes/develop

Changed getImage(..) to include tags in MachineImage response.
Fixed an additional bug in security group name validation (issue #16)
Merge branch 'develop' of https://github.com/greese/dasein-cloud-aws into develop
Added removeTags(..) support to AMI and EC2Instance.