More robust server specification

Also support a `beanstalk://` scheme URL for server specifications, so connecting to beanstalk servers via IPv6 literals work when listed in `config.php`.  Resolves https://github.com/ptrofimov/beanstalk_console/issues/59
I only wanted to match it if it starts with `beanstalk://`, since that's intended to be the scheme portion of the URL.  Explicitly checking for non-false will be satisfied by `beanstalk://` appearing anywhere in the string.