[REFACTOR] reverted wording Contact -> Buddy

Not every contact is a buddy ! This patch reverts only code changes but
not the messages that are displayed to the user.

Change-Id: Icb17efaea90e4bc915d8a734ad101ad9d4568206
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1160
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>