cvmfs_swissknife info can return current revision
`cvmfs_server publish` accepts (and checks) a manual revision number
`cvmfs_swissknife sync` can override repo revision
TEST: integration test 'manual revision number'
Merge pull request #466 from reneme/feature-manual_revision

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. Additionally this contains an integration test.