New structure of FairRoot repository

The "SVN" structure for FairRoot is changed. FairRoot here means the fairbase directory of the fairroot subversion repository. The release branch is freezed. It is set to read only for everybody and will not be updated anymore. Tags will be used from now on, the first tag v-13.03 is available. Experiments are highly encouraged to connect there code releases and trunk to tags, the fairbase trunk can also be used for testing new developments.