Static data describing an achievement.
Retreive the achievement descriptions by using Social.LoadAchievementDescriptions. GameCenter: These are stored server side and should be fetched when the user has authenticated. These are set in the iTunes Connect.
| achievedDescription | Description when the achivement is completed. | 
| hidden | Hidden achievement are not shown in the list until the percentCompleted has been touched (even if it's 0.0). | 
| id | Unique identifier for this achievement description. | 
| image | Image representation of the achievement. | 
| points | Point value of this achievement. | 
| title | Human readable title. | 
| unachievedDescription | Description when the achivement has not been completed. |