[REFACTOR] moved XMPP network classes to a dedicated package

Most of XMPP related features are now found in the net.xmpp package.

Change-Id: Id4a03aa98d51c77b5570f6ce7803c3a938d97874
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1607
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>