Pass fxa helper into FxaModel to simplify mocking when unit testing
Include Observable sources in fxa.html
Correctly use Observable. Naming is a bit clunky now.
Add a mock FxAccountsIACHelper for testing FxaModel behavior
WIP super sloppy progress

  * Mock FxAccountsIACHelper properly firing events
  * FxaModel catching the events and updating its Observable state
  * test successfully verifying that the Observable state is what FxaPanel and
    FxaMenu will expect

  Also:

  * figured out it's hard to "reset" FxaModel between tests
  * uneasy about conditionally wiring stuff up based on mozL10n
  * hope the rest of the tests go a bit faster :-P