Rewrite: Swissknife History Integration

This replaces the old history management in `cvmfs_swissknife` that was handled by `CommandTag` and `CommandRollback` by a couple of new commands, namely: `CommandCreateTag`, `CommandRemoveTag`, `CommandListTag`, `CommandInfoTag` and `CommandRollbackTag`. All of those are sub-classes of a new `CommandTag` that provided some common functionality. For example loading of manifest, catalogs and history databases as well as upload and publishing functionality.

Note: Adaptions for the integration test `520 - history` will come with the next pull request.

Note: This contains [Rewrite: Named Snapshot History Management Class](https://github.com/cvmfs/cvmfs/pull/591)
Yes, I was also thinking about this already. This whole spooler callback interface is pretty cumbersome for simple use cases in the swissknife. Adding a synchronous convenience API should be rather easy, but it would still need to spin up the whole TBB processing pipeline. One could think of a way to instantiate the whole processing in a serial mode, but that might require some deep thoughts. ;-)