iOS 7 styling

Here's a possible implementation of iOS 7 styling.

With this code, on iOS 7, the progress hud will default to a light, translucent, blurring background, with dark text.  The tint color can be changed using the appearance proxy.

This isn't totally done, but I wanted to get initial feedback on this, and see if anyone else is interested in wrapping it up.

**Issues**:
 - The "success" and "failure" images are only white, so we'll need black versions for this
 - Customizing whether the progress hud is translucent on iOS 7 is not implemented
 - I made the progress HUD larger to more closely match the ring switch / volume HUD on iOS 7, but now the layout code needs to be rewritten to center the content vertically

**Screenshots**:
![image](https://f.cloud.github.com/assets/789577/1017080/6f7eb496-0c08-11e3-8521-a7ad4e566001.png)
![image](https://f.cloud.github.com/assets/789577/1017081/74bbaad6-0c08-11e3-8b69-553e27199d29.png)
![image](https://f.cloud.github.com/assets/789577/1017086/7a5845a8-0c08-11e3-98b0-f243936559e5.png)