parsing hostname from command line or env

[TheOrioli]

A small change that enables setting a hostname via the command line or environment variables.

The motivation for this change is that Heroku has unreadable hostnames, and that makes it very hard to determine what sets of workers are doing what. The change is backwards compatible, nothing changes for users who don't set the hostname variable. I would prefer to change something else instead of hostname, but resque only shows that value in resque_web.

Before

After