Change 2 object methods to class methods

Hello,

please change these methods

```
- (BOOL)userHasDeclinedToRate;
- (BOOL)userHasRatedCurrentVersion;
```

to class methods like this:

```
+ (BOOL)userHasDeclinedToRate;
+ (BOOL)userHasRatedCurrentVersion;
```

Otherwise you cannot access them (sharedInstance is private).
