Convert project to static library

I think it’s easier to use the code in a bigger project when it’s packaged as a static library. More rationale in [another pull request](https://github.com/johnezang/JSONKit/pull/66) and in [these](http://zmotula.tumblr.com/post/10157814684/how-to-modularize-your-xcode-apps) [two](http://zmotula.tumblr.com/post/13587706529/xcode-and-static-libraries-tips) blog posts.