This commit refactorss the check_pool method in pool_manager.
Specifically, each commented section describing a stage of check_pool is
broken out into a separate method and check_pool is simplified by
calling these methods. Without this change it is difficult to follow the
intent for or make changes to check_pool.
Additionally, a docker-compose file is added to make it simple to launch
an all-in-one vmpooler instance along with a separate redis server with
docker.
Prior to this commit the docs and examples used 'company.com'
(a real domain). This commit changes those occurrences to
'example.com', which is a IANA-managed reserved domain.
