Feeds etc

Add support for feed de-duplication, and a couple other goodies -- base64 tools and allow `${config['brooklyn.property']}` to be used in `setup.script.vars`
I'd prefer us to use guava utilities where available, unless there's a compelling reason to do otherwise - see http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/BaseEncoding.html#base64(), or `base64Url()` if that is more appropriate.