Working on #8. Use tabs.activateTab to access unsafeWindow.

In order to access the Javascript variables, I need to use
``unsafeWindow.<var>``. To do this, I have the panel to attach
some content scripts to the current active tab.
Fixed #11. Remove gmail.js from submodule.
Fixed #10, #8. Use unsafeWindow in gmail.js to look up JS variables.

Due to addon security, to read web page's JS variable I have to use
unsafeWindow to look up the variable.