This important pull request solves a number of shortcomings of the is-thirteen library

• Not enough dependencies
• No error handling
• No node-gyp dependencies
• Not enough dependencies
• No asynchronicity
• API too simple
• Not enough dependencies

With this PR, you now:

• Have lots of dependencies
• Have to install O/S specific dependencies
• Have a really weird API
• Is much more node-y because it has a callback
• Have to know whether the thing you're testing will invoke a callback, or will return an object as before
• Have to read the code to figure out how to use it

Oh, but it will OCR image URLs and figure out if it's in image of 13. Well, it will figure out if it's a really good clean image of 13. Maybe. If it's a PNG image anyway.