Switch to using tabs for implementation

[jeffgca]

We've noticed that page-mod implementations that use include: "*" can slow down Firefox a lot because the content scripts etc are injected into any document, not just the top-level document. Using the tabs module to inject into the top-level document only ( especially if you're injecting jQuery ) eases some of the pain.

[jeffgca]

I should add, that using tabs removed the possibility of targeting an iframe.

[rlr]

Awesome, thank you!