Bug 1001833 - "TypeError: can't access dead object" due leaks of event/dom and event/chrome modules

- Ensured listeners and observers are removed once the add-on is disabled / uninstalled
Merge pull request #1476 from ZER0/dead-object/1001833

fix Bug 1001833 - "TypeError: can't access dead object" due leaks of event/dom module r=@jsantell