Convert to SDK-based add-on and simplify the code. Make it work with e10s.

Hey Greg,

I love your add-on and noticed that it unfortunately [behaves weird](https://bugzilla.mozilla.org/show_bug.cgi?id=1072821) with e10s. I took a look at the code, started to fix it, and would like to merge the following changes:

1) I converted the whole add-on to be SDK-based to be able to use the nice SDK APIs. That's what the first changeset does.

2) The second changeset uses the SDK's WindowTracker API to attach/remove event listeners to/from opened and closed windows. I seized the chance to convert the code to use two event listeners per window instead of having to maintain them per-tab. We can simply walk up the DOM tree until we find the tab the user clicked on.

I did not change the behavior of the add-on, it should still work the same. I however did add myself to the list of contributors if that's okay :) I tested the add-on in Firefox 32 and Nightly (35) and it works fine.

Should you wonder why I'm using a "deprecated" API (sdk/deprecated/window-utils) then that's because the new window/utils don't offer a similar WindowTracker API yet. That's however tracked in [bug 833011](https://bugzilla.mozilla.org/show_bug.cgi?id=833011) and I assume the deprecated API will not go away before that's fixed.