PCL not well supported

When using Portable Class library (PCL), some features are not available On line 747, you have 

```
 currentlySubscribed.ForEach(sub => currentSubscriptions.Remove(sub)); 
```

Which does not exists in PCL.

You have to use:

```
 foreach (var sub in currentlySubscribed) currentSubscriptions.Remove(sub);
```

Greetings,
Hugo
