Spira.repositories in threads

After some pondering (and more experience using Spira in a RoR project) I decided against storing the global list of repositories in Thread.current. I really don't see a reason behind this. Anyone?

This change allows one to do "Spira.add_repository" per "config/environments/*" in Rails, for example. Which is good, I think.

Also, Spira.settings appears to be obsolete now, so I've deleted it as well.
Tests are included.