Fixes ring layer and customizes it for iOS 7

• Fixes https://github.com/samvermette/SVProgressHUD/issues/192
• Customizes color and thickness of ring layer for iOS 7
• Foreground color used is the UIWindow's `tintColor`, so it will match other buttons throughout the app
![screen shot 2013-09-13 at 9 30 35 am](https://f.cloud.github.com/assets/789577/1138984/0938d344-1c82-11e3-868e-e2c34527e7ec.png)

• Uses `preferredFontForTextStyle:` as default font on iOS 7