Allow configuring redis namespace through environment, default is "esque"

We rely on different namespaces to separate production/staging/development environments as well as multiple projects exchanging jobs through the same redis server, just to make things clearer and avoid accidental intersections. In this patch I made namespace configurable through app environment. Please let me know if you think it should be configured differently.

Also, I didn't say that in the previous pull request but I should've – thank you for your great work on esque, it's a really useful tool and reading through the source code is a breeze.