FIX: Code Review Remarks

This is supposed to aggregate the code review remarks of the following (stacked) pull requests:

* [Rewrite: Named Snapshot History Management Class](https://github.com/cvmfs/cvmfs/pull/591)
* [Rewrite: Swissknife History Integration](https://github.com/cvmfs/cvmfs/pull/592)
* [Refactor: cvmfs_server Named Snapshot Management](https://github.com/cvmfs/cvmfs/pull/593)
* [FIX: API Adaptions for the new History Module](https://github.com/cvmfs/cvmfs/pull/594)

Which saves me a lot of time in rebasing them on top of each other every time a fix-commit is added to one of them.