Bug 865489 - Cannot iterate Addon SDK-List with for..of

Added `iterator` method to `List` (deprecated version too), so that our SDK list (`tabs`, `browserWindows`, etc) can be iterated with `for...of`.
Merge pull request #986 from ZER0/for-of/865489

fix Bug 865489 - Cannot iterate Addon SDK-List with for..of r=@gozala