add app:adapter, FakeServer

Adds an adapter for `app` model that prepends `/accounts/:account_id` when POSTing to create an account.

Adds `FakeServer` helper for acceptance tests that makes it a little smoother to stub requests. A FakeServer is started/stopped for every test, and it complains when it receives an unexpected ajax request, which helped me identify a few test failures.

Remaining items:

 * [ ] load a user's accounts after login
 * [ ] group apps on /apps by account, wire "create app" button to create a new app for that account