rpc.js improvements: re-re-review

1. RMR, new transport for Safari < 4, Chrome < 2, and should work as a backup on several other browsers (the latter claim requires more testing).
  - Paves the way to sunsetting IFPC, in turn sunsetting the need for containers to host "active" relay file rpc_relay.html and configure it properly.

2. Refactoring transport code into separate "classes". This cleans up the code a bit and makes it possible to optimize later by emitting only the JS a given browser needs rather than JS for all techniques.

3. Early-message queueing. Makes it possible to call gadgets.rpc.call(...) in a container to a gadget before the gadget is initialized (or even rendered), with the call still going through when the gadget is ready. It also ensures that G -> C calls made before G -> C initialization is complete also succeed (this is much less prevalent).

4. Various cleanups including tentative backward-setup support. As a convenience and to prevent user error, makes it possible to call gadgets.rpc.setAuthToken(gadget, token); before the gadget IFRAME is rendered. While not recommended in any case, this is intended to make the library easier to use.