getSubscribers does not work

There is an error in the _rtcclient\workitem.py_ when calling **getSubscribers**. The method calls resource that does not exist.
This line calls:
```
return self.rtc_obj._get_paged_resources("Subscribers",
workitem_id=self.identifier,
page_size="10")
```

However, in _rtcclient\client.py_ work item does not contain **Subscribers** but it contains **Subscriber** :
```
workitem_required = [
"Comment", "Subscriber", "IncludedInBuild", "Parent", "Children",
"ChangeSet", "Attachment"
]
```

running getSubscribers() results in:
```
raise exception.BadValue("Unsupported resource name")
rtcclient.exception.BadValue: Unsupported resource name
```