Text alignment and accessory views on buttons

Hi Jared,

This adds two things
- The ability to add a text alignment to the theme that applies to both the action sheet header and the buttons.  This was pretty straightforward.

- The ability to add an arbitrary UIView via a subclass of JSTActionSheetItem to the right of any button.  I chose to subclass the button view as well, so the existing API works as before.  The action sheet itself uses class of the item to know which type of button to create.  

The result can get you something along these lines:
![image](https://cloud.githubusercontent.com/assets/583236/4297541/6b7cb468-3e0e-11e4-8cd4-dfd7a9220079.png)