IGearClient.add accepts environment variables - fixes 568

I've tried to sketch out some code and made some notes for discussion during design review.

Fixes #568 
1. Yes, we do, but presumably it can be separate new issue in the Application Configuration Changes (Minimal) milestone (it's certainly out of scope for this issue).
2. I doubt there's any way to change the environment of a different running process; I know of none, and all the suggestions here (http://stackoverflow.com/questions/205064/is-there-a-way-to-change-another-processs-environment-variables) are hacks.