Migrating old Repositories to current version

* This checks if a repository is compatible to the currently installed CernVM-FS version and warns the user in case of incompatibility.
* Furthermore it allow to update the repository by calling `cvmfs_server migrate`
* From now on **server.conf** contains `CVMFS_CREATOR_VERSION` to make the migration process easier in the future
* `cvmfs_server list` shows the version of the creator when it differs from the current version
* migration should be flexible enough to work transitively: 2.1.6 -> 2.1.7 -> 2.1.8 in one run

Known issue: It can only handle 2.1.x repositories. Migration from 2.0.x is not supported atm.