Issue #615 Fix -Titlebar with only one button has text out-of-place

Authors (github usernames)
@haixuanc
@williekwok

Changed the margin addition to 0px for title bar caption when there is only one button (either the back button or action button). 

Tested in both simulator and device
![Screen Shot 2013-01-19 at 10 34 16 AM](https://f.cloud.github.com/assets/1329555/80517/33b68e66-6267-11e2-88d8-4ecd62a7b5f7.png)
![Screen Shot 2013-01-19 at 10 35 07 AM](https://f.cloud.github.com/assets/1329555/80518/33c7eb3e-6267-11e2-91ed-b80a3f9eaa39.png)