Plugin Multi-process architecture

I've started documenting the Plugin+Electrolysis architecture here:

https://developer.mozilla.org/en/Plugins/Multi-Process_Plugin_Architecture

As soon as we rename the plugin protocols the way we've talked about, I'll
flesh out a little more of the docs about the particular protocols and what
they represent.

Feel free to correct or add as appropriate. If the diagrams are incorrect or
you think they need more information, let me know: I have the Omnigraffle
originals on my local computer.

--BDS