Properly handle projections where '_id' is the only rule.

+ Tests. Fixes #1651
Fix 0.6.6 regression in setting MAIL_URL

Also:
- ensure that AppConfig callbacks are always called at least once
- don't start a new AppConfig Fiber every time you call Email.send

Fixes #1649.
Merge branch 'devel' into oplog

Conflicts:
	packages/application-configuration/config.js
	packages/minimongo/minimongo.js