Framework compatibility

OpenSans now works when it becomes a framework with CocoaPods 0.36.

Two steps were necessary (based on [this CocoaPods blog post](http://blog.cocoapods.org/Pod-Authors-Guide-to-CocoaPods-Frameworks/)):
- Used a system import (`<OpenSans/UIFont+OpenSans.h>`) to import the header.
- Created a private class to load the font, as `NSBundle +mainBundle` needs to be replaced with `NSBundle +bundleForClass:`, but couldn't do it from a category.