Fix layout issues

There are layout issues when running the SDK on iPhone 6 and iPhone 6+ screen sizes at native resolution. The sign in button on the traditional sign in view controller has a gap on the right side. The loading screen that is displayed when signing in is not correctly laid out either. This commit fixes these issues, but there are likely others.
![ios_simulator_screen_shot_dec_10__2014__2_45_17_pm](https://cloud.githubusercontent.com/assets/10146366/5384402/a60dd7da-807e-11e4-82ba-1b33eb5efe7e.png)
![ios_simulator_screen_shot_dec_10__2014__3_07_46_pm](https://cloud.githubusercontent.com/assets/10146366/5384451/16463cea-807f-11e4-9dcf-618e26cff840.png)