Resolve bug where XHTwitterPaggingViewer was incorrect height when inside a UITabBarController

Hey,

I fixed a bug that was causing my scrollviews to have an incorrect height when displayed in a UITabBarController.

Before my fix:

![xhtwitterpaggingviewer_before](https://cloud.githubusercontent.com/assets/296341/3836736/723d8b4a-1ddb-11e4-877c-8bb51a1998e2.gif)

After:

![xhtwitterpaggingviewer_after](https://cloud.githubusercontent.com/assets/296341/3836738/74d85a42-1ddb-11e4-82d9-ab1e53a8ffa0.gif)