Enable queued responses from stubbed API and add more tests to Login

This branch adds the ability to queue multiple requests using the stubbed `AsyncHttpClientStub`.  It uses them to go from the `LoginActivity` to the `MainActivity`, for example, which causes two HTTP requests.

Also, this Robotiumifies the `LoginActivity` tests and adds tests for clicking the Register button.

*Edit: Now with Robotiumified `RegisterActivity` tests!*