Changed plugin methods for Cordova 1.9 compatibility.
Fixed data argument not used by format string warnings.
Changed plugin callback system and authentication.

- Removed automatic authentication.  Now users of the plugin must explicitly
  call authenticate providing success and failure callbacks.

- Changed callback system so that users of the plugin can explicitly provide
  callbacks to be invoked for authenticate, reportScore, and getAchievement.

- Added onshow and onhide properties that client code can use to register
  functions gamecenter show/hide callbacks.
Renamed method getAchievement to reportAchievement.
Fixed bug with setting leaderboard category.
Merge pull request #1 from wcwynn/v1.9

Implemented Cordova 1.9 compatibility.