[REFACTOR] renamed InvitationProcess class

Renamed InvitationProcess to SessionNegotiation to be conform with the
already existing ProjectNegotation class.

Change-Id: I376df38a6279834609cd0b47e71196195f9310b8
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/925
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>