Implement addon/bootstrap shared module.
Read module paths from preferences.
Fix path reading.
Do not nuke sandboxes on unload as that could kill sandboxes where bootstrap is loaded.
Remove pref for mountURI
Merge remote-tracking branch 'upstream/master' into bootstrap
Add a bootstrap.js for testing.
Fix the last breaking test.

Note no need to cache loader module as caching is built-in into loader jsm itself.
Merge pull request #1742 from Gozala/bootstrap

Bug 1075541 - Implement addon/bootstrap as shared module. r=@erikvold