Tanything's prompt as emacs' `C-x b`

Hi,

This PR (if merged) permits to Tanything's prompt to work in a similar fashion as emacs' `C-x b`.

From a user point of view, the selected tab will be the but-last visited one.

From a developer point of view, it introduces a dependency on mozilla's add-on sdk - https://github.com/mozilla/addon-sdk/ - on tabs api to have access to tab events. 
This way, we can add behaviours in reaction to these events without impacting the initial code.

I used keysnail's standard way of dealing with dependencies - https://github.com/mooz/keysnail/wiki/Writing-Plugins#wiki-require.

The initial discussion is https://github.com/mooz/keysnail/issues/95.

Hoping you'll enjoy this enough to accept to merge it.

In any case, thanks for Tanything.

Cheers,