add support for defining services (composite of groups)
restructure the DSL to better reflect the nested nature of it
add CLI interface for describing a service

- switch to thor for the option parser now that the CLI is getting more
  sophisticated
update README
ensure the default CLI task is apply
note the enhancement in changelog
fix readme
add an API for querying the groups of a service
fix typo
require that groups referenced by services are defined
don't break the -v option
Merge pull request #2 from undeadlabs/services

Service Definitions