Added support for autolayout and refactored internal code.

We've made some updates to the badge for use in our own software.
- Added documentation comments to public methods (which will show in XCode 5+).
- Updated badge to handle auto-layout better.  It now has its own intrinsicContentSize which auto-layout will respect.
- Fixed issues with partial drawing of the badge when UIKit decides to invalidate only a part of the drawing rectangle.
-  Support for being added into `.xib` and `.storyboard` files.  Still can't edit properties of these in Interface Builder, but you can at least place them and setup constraints.
- Updated to not use deprecated methods in iOS 7 like `sizeWithFont:` and `drawAtPoint:`.