Clarification whether XmppSession is thread safe.

[Markus KARG]

There should be a clarification in the XmppSession's JavaDocs whether it is safe to use one XmppSession instance from multiple threads at the same time or must be serialized explicitly, and it should be told whether all access has to be done using always the same thread (like in Swing or JavaFX). Only the implementor of the Babble framework can know this, but it is essential for the application programmer when using Babbler in a multi-threaded application.

[Christian Schudt]

Add more JavaDoc for XmppSession class.

Fixes issue #11.

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