Rewrite: Named Snapshot History Management Class

This is a rewrite of the named history database wrapper that was called `TagList` before. Now this responsibility is fulfilled by the `History` class. It fully hides the SQLite details from the using code. Hence, all database queries and the SQLite connection is handled by `HistoryDatabase`. Note, that this Pull Request does not contain necessary API changes in the user code and hence will break the build when merged alone.

Additionally there is a unit test set for the `History` class.
Okay. [Here](https://github.com/reneme/cvmfs/commit/fb4a23d67574010ac934d2afc1f76256edab04cb) you go.