Allow list of servers to be set from environment

If BEANSTALK_SERVERS is set in the environment, explode on commas and
add the resultant elements to the list of servers
Also, avoid a PHP undefined variable warning when there are no groups
setup by setting $groups before including templates
mention BEANSTALK_SERVERS env var in README
actually fill in variables that should be set
Merge pull request #61 from thwarted/server-list-from-environment

Server list from environment