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)
A similar check used to be in the old code, I just adapted that. Anyway, an assertion should suffice, that's true. [See here](https://github.com/reneme/cvmfs/commit/7625e78d4e6d16af928324fe60e7135e11e60b98).