Issue #15: Add-on is enabled but doesn't load when Firefox upgrades

Watch all windows that haven't loaded to check if it becomes a navigator:browser as the watcher's notification is too late and mediator's enumerator doesn't match.

Upstream issues:
BarTabLite: https://github.com/philikon/BarTabLite/issues/3
Prospector: https://github.com/mozilla/prospector/issues/442
Prepare 1.6 release
Update version
Added tag 1.3 for changeset c617ba9657cb
Added tag 1.4 for changeset 1b7841f52204
Added tag 1.5 for changeset 41c503462e13
Added tag 1.6 for changeset b3f0e5a6c66c