Added copy of third-party library JSONKit

NOTE: JSONKit is by John Engelhart, c. 2011, Apache/BSD License.
      http://github.com/johnezang/JSONKit (commit 82157634ca)
NOTE: Did not add as a submodule as modifications are needed in order to
      compile it with the latest SDK.
Updated JSONKit to allow for compilation

NOTE: Made the following modifications:
      * Do not directly access isa pointer.
      * Ignore "introspection" warning.
      * Added prefix "JK" to JSONDecoder.
Added method to init instance with JSON string
Moved common implementations to PIPackage base class
Use NSJSONSerialization for iOS 6.0+

NOTE: NSJSONSerialization is also available in iOS 5, but various reports
      indicate that iOS 5's implementation is unreliable.
Methods initWith* must be publically accessible
Refactored debian package's install date method
demo: Link to library instead of compiling own version
Use standard set of key names for JSON data
Make packageDetails_ mutable, use to cache values