Feature: Setting Repository Revision Manually

This allows the manual definition of the next repository revision number by means of `cvmfs_server publish -n <new revision>`. Unfortunately the `-r` switch was already taken. It is not possible to set the revision to a smaller number than the current repository revision, of course. This feature was requested in [CVM-633](https://sft.its.cern.ch/jira/browse/CVM-633).
Additionally this contains an integration test.