NUX visual and UX improvements

This is phase 1 of a larger restructuring of the sign in / up process and the first launch experience. It is mostly a visual update with some UX improvements as well.

Related issue: https://github.com/wordpress-mobile/WordPress-iOS/issues/348

![ios simulator screen shot nov 14 2013 9 58 42 am](https://f.cloud.github.com/assets/3904502/1543424/64fe4cc2-4d56-11e3-8cb7-c8d6c0b58416.png)
![ios simulator screen shot nov 14 2013 9 58 46 am](https://f.cloud.github.com/assets/3904502/1543425/651afe9e-4d56-11e3-8dda-0a95c8f39323.png)