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 think is better to check
```php
if(!strpos($server, "beanstalk://") !== false) {
```