A few notes about this:

* I didn't want to `include Resque::Logging` as a) we don't need the different severity levels and b) those method names are super-common and would squash any jobs that do something similar (like my jobs :)
* Checking for `Resque.logger` is mirroring what's in `Resque::Worker` - presumably there is a good reason, but I guess generally logging should not take down your system
* The environment variable is there so that a released version of this library would behave the same way, *just* in case the addition of logging causes some problem.  This means people can opt-in, *and* they can opt-in on a per-worker basis, which seems handy.  The value of `"true"` is debatable.  I prefer it, since `ENV` can only store strings, but I know some people like `1`.  If someone feel strongly, we can change it.