Add GetParent() To PrivateMessage

Returns the parent of a privatemessage, if it's in a thread.
Update PrivateMessage.cs
Update PrivateMessage.cs
Update PrivateMessage.cs
Update PrivateMessage.cs
Add Parent property and GetThread() method

Because I'm a nice guy
Merge pull request #72 from Erok21/master

Add GetParent() To PrivateMessage