xmpp-client tests shouldn't rely on JavaFX

[Thomas Calmant]

The implied JavaFX dependency currenlty forces to install the Oracle JDK (at least on Ubuntu 14.04) to test xmpp-client. Maybe it should be considered as non-GUI library, therefore with non-GUI tests. The JavaFX tests could be executed in a separated artifact.

[Christian Schudt]

Thanks for reporting. I will keep that in mind and fix it.