[REFACTOR] renamed and moved ProjectExchangeInfo class

The name is a bit "nichts aussagend" ... Info can be anything. Afaik it
includes the data that is needed during a project negotiation. Moved the
class to the invitation package as the class has nothing to do with
activties. Also added XStream annotations as this class is marshalled.

Change-Id: I59e26412151afdb12924688bd971f86e105fb5c6
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1204
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>