TNKImagePickerController 3.0.0

A big thanks to Alexsander Akers on this release for all of his pull requests.

• Added nullability and generics annotations.
• Refactored auto layout constraints to use iOS 8 compatible API.
• Prefix category methods to prevent namespace collisions.
• Fix for asset selection persistence issues.
• Add ability to customize selected asset badge image.

Note that if you were using any of the extensions provided by this framework, they are now prefixed with tnk_.