Update login view and launch images

Screenshot on iOS 6.1:
![login view ios 6 1](https://f.cloud.github.com/assets/195837/1188578/7b7c8abc-23db-11e3-93f7-203542433fc2.png)


Points to look out for:

* The shade of green used in the Dropbox button is very bright
* iPad launch images are sized for iOS 7. They looked fine on the 6.1 simulator, but I don't have a device on 6.x to test on.
* Missing an @2x version of the Dropbox logo